Welcome to my guide for an AP Orianna.
This guide shouldn't stay static in the item build.
This guide should be changed to fit what you like. (Example- Summoner Spells.)
This guide won't always get you a win.

Pros:
Caster.
Ranged.
Can stay back and let her ball do all the damage.
Part support.
Can easily farm mid game.
Can easily make a quick getaway.
Cons:
Can be squishy.
Can't do much damage after her Q is used up (after using W,E,R.)
Lacks damage very early game.
Not dominating the lane at all during the game will set her back and probably cripple her from doing a lot of damage.

Orianna uses mana very fast and easy in the beginning of a game. I recommend using mana regeneration runes. Though, using ability power is an alternative. More mana is something you can use as well, though what is the point of mana if it is used up easily. Magic penetration is another thing you could possibly get, and is alright for once your able to spam spells in the most effective way. Cooldown reduction is the last thing you could get, it is another alternative and can be effective since you do need to spam spells.
Greater Mark, Seal, and Glyph of Replenishment: Mana regeneration is key to spamming spells, since you need some income of mana, otherwise there is no point in this build. Ability power and magic penetration can be another focus so added it to magic regen like I show in the runes can be another effective way.

Using utility allow for even more mana regeneration (plus health regeneration), mana, cooldown reduction, gold, and experience plus some improvements to summoner spells depending on what you choose. Using offense allows for magic penetration, ability power, and cooldown reduction for more of a chance to spam abilities. You shouldn't use defense unless you for some reason are going to be a little more of a tank or use heal or revive. Putting some points in utility master can be more effective than the 2 points in awareness if you get buffs often.

The item build I show is a standard one. It SHOULDN'T be used as is, unless you can stay alive easily and do a lot of damage.
Early Changes: A start you can do instead of a Doran's Ring is a Meki Pendant and some pots. Another thing you can start with are some pots with Boots of Speed, however early AP is what you really need unless you want more mana regeneration along with more lane sustaining. The Ionian Boots of Lucidity can be switched to Sorcerer's Shoes if your enemies are mainly tanky/ have magic resist or to have a bit more power in your abilities.
Morello's Evil Tome: Cooldown reduction, ability power, mana regeneration. What more could you need? Once you get this, your damage output should kill minions with a Q->W.
Ionian Boots of Lucidity: Even more cooldown reduction means more spell spamming!
Mid Changes: You should put something between the Tear of the Goddess and completing an Archangel's Staff like some magic resistance or armor if your having trouble surviving, or just want extra protection. Completing an Archangel's Staff isn't too necessary unless you stay out of your base for long periods of time, so if you go back a bit often you can speed up the process on a Void Staff. However, even if you do go back often getting an Archangel's could be a bit better so you can stay out of your base a bit longer depending on if you go back for health or mana. (Mana=better.) (Health=Armor, Magic Resist, or Rylai's Crystal Scepter) If you don't want to go for a Void Staff, you should upgrade any armor or magic resist items to an Abyssal Scepter for more damage and MR or Zhonya's Hourglass for the active (if your in trouble) plus the damage your getting from the ability power.
Tear of the Goddess: A bit necessary mid game since mana runs out slower than early game, but fast even though. Spam abilities when you return to base a few times for some mana, and make it to an Archangel's early or late depending on how your mana depletes.
Archangel's Staff: Not as necessary as a Tear of the Goddess depending on how much your mana depletes.
Late Changes: If you didn't want a Void Staff you should already be upgrading for some armor or magic resistance, or already have it depending on how you farmed and how you did or didn't dominate your lane.
Rabadon's Deathcap: More ability power means more damage. What more could you ask for?
Late Additions: If you continue on after a Rabadon's Deathcap, head for more ability power since you want more damage.
Void Staff: I usually get one, since there is some kind of magic resistance usually. The added ability power and magic penetration should help against tanks, and especially other AP casters since they will be pretty weak to your damage just like you may possibly be weak to their damage. However, if you get Sorcerer's Shoes you might want to make your way to Rabadon's first, since you would want more damage with the penetration. Though, getting a Void Staff with Sorcerer's Shoes is still a good choice.
Within Game:
Health Potions: If you struggle to stay in your lane, health pots should be something you should buy, unless you can finish an item like Morello's Evil Tome, since your damage may back them off of you.
Mana Potions: If you really want mana early game, this is something you want to get, however health pots should be your focus since you really shouldn't be harassing early on unless you have clarity.
Wards: Use your ball instead of wards, or clairvoyance to keep some map control, since getting your items are more important than wards. If you do feel that you need them since you get ganked often then you should get them, otherwise it shouldn't be your focus.
Optional:
Frozen Heart: More mana and armor plus cooldown reduction is great. However make sure you have some mana regeneration to go with it plus a good amount of AP for damage.
Banshee's Veil: Magic resist, more mana, more health. It will help you survive even longer, though make sure you have some AP and mana regeneration.
Rod of Ages: Not that best starting item, since you won't be able to spam your spells for some time still, since where is your mana coming from? (Other than Doran's Ring or Clarity)
It can still be very great if you manage to farm as well as you would with the items I show, though I don't find it to be the case since you can't support harassment as much or farming a lot either.

Clockwork Windup - Orianna's autoattack deals additional magic damage. This damage increases the more Orianna attacks the same target.
Orianna's passive isn't too useful, though if your spells are on cooldown, it can help deal some extra damage.
Command: Attack - Orianna commands her ball to fire toward a target location, dealing magic damage to targets along the way (deals less damage to subsequent targets). Her ball remains at the target location after. Orianna commands her ball to shoot toward a target location, dealing 60/100/140/180/220 (+0.6) magic damage to targets along the way. However, it deals 10/10/10/10/10% less damage for each unit it hits (Minimum 40/40/40/40/40%). Her ball remains behind at the target location afterwards. Cost4 0/45/50/55/60 Mana Range 900
Her Q is a critical ability. It controls your W and R. So getting this ability first is needed. This still will help you get last hits at level 1, and harass on level 1.
Command: Dissonance - Orianna commands the ball to release a pulse of energy, dealing magic damage around it. This leaves a field behind that speeds up allies and slows enemies.
Orianna commands her ball to release an electric pulse, dealing 70/120/170/220/270 (+0.6) magic damage to nearby enemies. The pulse leaves behind an energy field for 3/3/3/3/3 seconds, lowering enemy Movement Speed by 25/30/35/40/45% and increasing ally Movement Speed by 25/30/35/40/45% for 2/2/2/2/2 seconds. This effect diminishes over time. Cost 70/80/90/100/110 Mana Range 225
Her W is your main way to harass mid game/late game, or once you get Morello's Evil Tome since the mana cost can be controlled more easily. Always harass if your enemies are low, and try to get a kill with your R if they will be killed by it.
Command: Protect - Orianna commands her ball to attach to an allied champion, shielding them and dealing magic damage to any enemies it passes through on the way. Additionally, the ball grants additional Armor and Magic Resist to the champion it is attached to.
Passive: The ball adds 10/15/20/25/30 Armor and Magic Resist to the allied champion it is attached to. Active: Orianna commands her ball to travel to and attach onto an allied champion, shielding them for 4/4/4/4/4 seconds from the next 80/120/160/200/240 (+0.6) damage. Enemies the ball passes through along the way are damaged for 75/75/75/75/75% of the shield value: 60/90/120/150/180 (+). Cost60/60/60/60/60 Mana Range 1020
Her E isn't really useful till late game when it is at level 5, since it can soak up a good amount of damage. Unless your going to go full support, putting one point into this should be good enough till late game, since damage should be your main focus most of the time. (Obviously saving an ally is important so do what is necessary to save them, except getting yourself killed.) Remember that even though the shield runs out, the allied champion that ball is attached to, gets armor and magic resistance.
Command: Shockwave - Orianna commands her ball to unleash a shockwave, dealing magic damage and launching nearby enemies towards the ball after a short delay.
Orianna commands her ball to unleash a shockwave after a brief delay, dealing 150/225/300 (+0.7) magic damage to nearby enemies and flinging them into the air a set distance in the direction of the ball. Cost 100/125/150 Mana Range 410
Her R is a pretty good ability. It can be both offensive and defensive, and can turn the outcome of a fight. Offensive: Always try to get 2 or 3 enemies in your R if your in a team fight. The damage will be pretty good, leaving your allies to do even more if they are running as well. Defensive: If your teammates are low, using this ability will be useful in making sure they get away. Always make sure if you want them to get away from you the ball is behind them, and then press R to send them back. Always make sure if you want them to come to you the ball is toward you and press R to send them towards you for a kill possibly.
Do NOT harass very early on unless you get mana potions, don't have someone that can give you mana (Soraka), or need to stop your enemy from farming. (Stopping them from farming should only be done when you have some extra mana regeneration since your mana drains too fast.)(You can harass as long as keep a decent amount of mana around for creep kills, so you have a W incase there is an early gank. Harass/Farming Combo: Q->W
Use this for getting minion kills. However don't harass early on. Waiting till you get a Fiendish Codex should be good enough. Once you get Morello's Evil Tome you should harass a bit.
Survivial Combo/Chasing Combo: (Yourself or an ally.) E->Q->W or Q->W->E or Q->R->E->W
Use protect first if the ally is low, or is taking a lot of damage, or will be, otherwise use the Q->W combo first to slow enemies and then use protect so that you get more of a chance to damage your enemies. The last possible combo is Q->R->E->W, which can be switched around depending on if your ally will die if you don't use protect and speed them off first, though saving an ally is a part of your job late game, since you have the mana to do it, and protect soaks up a good amount of damage. So using this combo should be used to get off damage but also slow the enemies down in catching up to you if the ball is behind them.
Chasing/ Chasing Combo: W/E->W
If your ball isn't right on you or an ally, use protect first on yourself or the ally then use W to speed them up to possibly get a kill.
Low HP Enemies Combo: Q->W->R
If the enemies are low enough, using this combo will kill them off if they take enough damage and don't dodge your abilities.

Full Damage Sequence: Q->W->W->Q->W->R->W->Q->W->Q->R->Q->E->E->E->R->E->E
Damage with Support: Putting protect into the sequence will make you part support early on, which I usually do, since you might be taking some harass. So if you want to put some points in protect, you can possibly do it at level 3,5,8, and by level 10 you should have at least one point if your going to put a point in. After that you can throw in 1 or 2 points within there then go back to Q or W. I usually put 1 point in a bit early (level 5 or so) just so I get some protection if I get ganked, though having more than 2 would be too much till about level 13, since you want more damage.

Recommended:
Ghost: While using ghost and your W, you will just speed away from trouble. Though if your already being chased, it may be a bit harder. However, as long as you aren't slowed, you should be able to escape. (If stunned or suppressed, you may be able to also as long as you survive the damage that comes at you.)
Flash: Flash and W is just as good as ghost, but will sometimes be better since you can go over walls or dodge things like Ashe's arrow or Nidalee's spear.
Teleport: Mobility is key. However, this spell isn't totally necessary. Though it is useful to go to lanes easily, especially if you have protect, since you can easily help an ally.
Clarity: Mana problems are common. So if you really want to harass and dominate your lane, this can be a key item.
Possible:
Clairvoyance: If you want some map control, along with your Q, this is for you.
Exhaust: A great spell to slow down enemies and possibly get the on them.
Ignite: Can help you get kills, especially if your enemies focus on healing. (Ex- Taric.)
Don't Get:
Heal: Not really necessary, since mana problems should be more of your focus, since you could just buy health pots if you really need health.
Revive: No point to this spell. If you die too much, you should be buying armor or magic resist. If you still die, make sure you stay as far back from possible fights as possible so you don't get focused. (Make sure you still do damage even though your backing away a bit, otherwise your not helping do your main focus. (DAMAGE!)
Smite: Leave this for junglers. Unless you really want to try jungling, this really is useless. (Don't try jungling... Orianna should be in a lane.)
Cleanse: Not needed. If you are worried about slows, taunts, stuns, etc, stay back even more than you usually would and avoid them.
Fortify: Leave this for tanks or true supports (Soraka, Sona, etc) since there are better spells.
Rally: Unless your going AD Orianna this is useless.

Orianna really needs to farm. If she doesn't she will be set back in getting items, leaving her damage to be set back as well. So, if your support is farming while you are at the lane, tell them to not do that, even though they shouldn't anyway, since you need gold more than they do.
Early: Farm using your abilities a bit with your auto-attack, and don't harass till you get Morello's Evil Tome at least. (Unless you have clarity or mana potions.)
Mid: ALways use your abilities to farm while using your auto-attack. Harass a bit while doing this if you can.
Late: Always use your abilities to farm. Make sure you keep farming up when possible, and harass when it is safe to.
Don't: Push your lane too much unless you need to go back, otherwise you should do this so your tower won't be pushed/pushed very much.
Harassing: Q->W

Top or Bottom: Laning with someone who can help you harass, or damage is key to dominating your lane. Having a support can be good as well, though its more like you have to do everything yourself depending on what support you have. You can buy wards if you feel that they will be necessary, though buying your items is more important.
Mid: You should try to mid if you can, though it isn't necessary. However, being a main damage character on your team, you should mid since, spotting ganks are easier than top or bottom since your ball will spot them usually in the brush. Also, having more experience and gold can put you over the top, and help you dominate your lane even more. If your having trouble laning, always try to get a gank, to at least push your enemy back so you can farm a bit. Having an AD enemy seems to cripple Orianna if you can't get them to back off with some harass, so try to get a gank or get someone else to mid if you come across this problem.
Solo Top or Bottom: This can be hard for Orianna at first, since she doesn't do too much damage and because of that farming will be hard at first since your enemies will be all over you if it is a 2v1 since it will be easy for them if you get too close. So try to get a 1v1 solo lane on bot or top, go mid, or take a 2v2 lane if this gives you trouble.

If you lane with someone, try to make sure that they will help you harass and do damage (unless they are some supports). So people like Swain, Veigar, Xin Zhao, or Tristana can help you. Having Taric or Kayle as a support character can also be good, since Taric has his stun, and Kayle can slow, and do some damage while making sure that you take no damage if her ultimate is on you.
Try not to have an AD character as an enemy, unless your ready to deal with them since they really can cripple Orianna. So champions like Ashe, Tristana, Wukong, or Xin Zhao can make farming harder than usual. Having a tank can help you with this problem, since they hopefully could soak the damage for you.

Overall, AP Orianna is a good build. While using this build, I have won a lot of my games.
Always know when to harass, and how far to push a lane. Make sure you make this build into something more that you like, such as different summoner spells or runes too.
Please make any tips on how to improve this build/guide as this is my first one.
Make sure you comment and vote! =)
-Woody23-