Welcome to my Orianna guide. I will be a little more detailed in my guide. This is how I like to play her and build her. Please read entire guide and leave comments to help me improve! Thank you!

Changelog:
June 6th - Worked on my runes!
June 5th - Fixed some spelling errors and added a bit more details. Runes work in progress.
June 4th - Published, still need to work on Runes and situational items.
June 2nd - Guide was made

Still working on it! =)

Pros:
Very good at zoning and harassing
Great farmer
AOE AOE!
Unmatched ult

Cons:
Squishy
Dependent on ball's positioning
Need allies to understand her abilities
Mana problems early game

(Passive) Clockwork Windup
A great innate but difficult to know how to use it. It's good to have and useful when, for example, you're out of mana or harassing with AA, but not something you would focus you're build around. Items like Malady and Nashor's Tooth are okay items if you prefer to build your Orianna that way, but I prefer to focus on using her ball to deal damage.

Remember that harassing with your auto-attacks is effective thanks to your passive. You have relatively good range so abuse that and zone out your enemies.

(Q) Command: Attack
Pretty self-explanatory. This ability allows you to position your ball. Being on such a short cooldown, you can easily abuse it and harass your opponents. Another thing to note about Command: Attack is that it damages enemies that it passes through. So you don't technically have to land the ball on them, but it's a good idea to so you can activate your Command: Dissonance.

Things to note about the ball. You get the ball back by either moving away or picking it up. Keep an eye on how far you are from your ball. At times you might just walk out of its range and mess up your cast. Other times you might be in it, but your casting Command: Attack... The ball will actually travel the entire distance. Also, the ball can be used to check brushes. You can also cast it through walls.

(W) Command: Dissonance
This ability is actually your main damage in my opinion. The slow and speed is very useful. Remember that this actives wherever the ball is and thus you can activate this with whichever champion the ball is attached to. Another thing to keep in mind is that this ability also speeds up your minions.

You can activate this ability to help you and your teammates get to your lane faster. Simply press W =) With the blue buff/elixir, your W is almost like Zilean's Time Warp. You can move around map with ease.

(E) Command: Protect
This ability gives a very powerful shield as well as a buff in both armour and magic resist on a target ally (as well as yourself). Use this to shield your ally in a tower dive, your squishy who is being focused and even your tank who is initiating. The damage the spell does isn't the best, but still useful. Position yourself and cast it on yourself to do some extra damage.

Command: Protect is similar to Command: Attack in that it help you position the ball. You can use it on you initiating ally such as a tank and then activate W or R to do a bit more damage. But this also means your teammates must keep an eye oh your ball and understand what it does. If you place it on them, to activate your ultimate, they need to also understand not to back off, causing you to miss. One thing to be careful with Command: Protect is when your teammate die, the ball returns to you. So if you attached your ball in attempt to save an ally, activating either Command: Dissonance or your ultimate, you might waste it by accident.

(R) Command: Shockwave
A beautiful ult. They want to run? NOPE. They group together? DAMAGE. They're channeling? STOP! You want to use this mainly to help disrupt the enemy team rather than for the damage. Note 'mainly'. During situations of potentially getting multiple kills... I don't see why you won't use this =)

Firstly never put your first skill point immediately. Most of the time I will put into Q, but in cases of level 1 fights etc. I might need to put the first point into W for the slow and haste. Furthermore you might consider putting your first point into E to help your jungler. Endless possibilities.

Many would suggest maxing out Q immediately by level 9. I beg to differ. The reason I alternate Q and W is because W is your real nuke. Remember that Q will do reduced damage for each additional unit hit. Your standard harass combo is Q then W right after, assuming you've hit your Q that is. The Ball is highly unpredictable and this is why you are so powerful zoning out and harassing your opponents. You want to surprise your opponent, attacking with the ball and then activating dissonance doing a significant amount of damage. Therefore sometimes you might just cast it through a wave of minions. It's Command: Dissonance that will be doing the damage then.

As for E, the reason I put a point in E early on is to aid me in returning the ball in times of need. Also it gives nice amount of defense stats are necessary for Orianna. Remember that her base Armour is 8 which is ridiculously low. I normally max this last because the damage it does isn't that great.

Flash is a necessary in my opinion. It's great for both escaping and running away. A great way to use Flash offensively is imply flash into your opponent, activate Command: Dissonance and follow up with Command: Attack and so on.
Ghost is also another summoner that I normally pick up. I don't think this is as necessary since your W is already such a good escape mechanism. Nevertheless, I think having an extra ability to escape or catch up to enemy is great. I normally have this because I play mid with Orianna. This helps me escape potential ganks.

Other good ones:
Ignite is useful, finishing off those who manage to run away from your ball, as well as reducing a bit of healing. I use this for a bit more offense in my games.
Clarity is only really for those who wants to harass endlessly. I haven't had a trouble with my mana early game, even without runes. Zoning and harassing must be well timed and planned, not just constantly throwing your ball at them, using Command: Dissonance and missing. But Clarity is good to grab if you need want.

Greater Mark of Insight
Standard AP runes for some nice Magic Penetration. Useful in every way.

Greater Seal of Clarity
Mana regen helps sustain you in lane. You will have mana problems, so again this will help you lane.

Greater Glyph of Force
AP is always good. Makes you hit harder. And that's what we want.

Greater Quint of InsightGreater Quint of Insight
More magic penetration!

Core:
Rod of Ages - Orianna is squishy, and this provides her with a bit more health and mana without too much sacrifice of AP. In a way, I'm building her a tanky AP caster. I like to stay alive in my games and this helps me greatly.
Rabadon's Deathcap - Pretty obvious what this does. Good for all your skills, useful in every way. No matter how you build her, you want this item.
Lich Bane - Having so much AP, your AA will definitely hurt with your passive. The little movement speed is great to catch up with your ball and the magic resist is very welcomed.

Which boots?
Sorcerer's Shoes - These boots are great for caster. Any magic pen is wonderful in my opinion.
Ionian Boots of Lucidity - I don't suggest these (explained below) but for those who prefer a little guaranteed CDR, you can pick this up.
Mercury's Treads - Even with the addition of tenacity items, I still prefer Merc Treads. Pick this against heavy CC teams.


Situational: The last two items you get will need depend on how your game is going! So feel free to change the two I indicated above.
Zhonya's Hourglass - More AP! Armour! Great item to pick up. I normally get this item if the game drags on that long. The AP is welcome and the unique active increases my survivability.
Void Staff - When they start stacking MR seeing your great damage, you might want to pick this up.
Banshee's Veil - The bubble is always good. In the case of a
Morello's Evil Tome - Again, similar to Ionian Boots of Lucidity, if you prefer some guaranteed CDR and purchased Sorcerer's Shoes or Mercury Treads already. Again, read below why I do not get CDR items.


I don't get:
Tear of the Goddess/Archangel's Staff - Mana is a problem with Orianna early game, but I don't think the solution is Tears of Goddess. I solve it with my Catalyst. Also her abilities are still relatively high on cooldown, even with max cooldown reduction so you probably won't even max the Tear early enough until it becomes effective. Leave this item for Ryze or someone, not Orianna.
Rylai's Crystal Scepter - I've never been a fan of this item and I don't suggest it for Orianna even more. Her abilities are multi-targeted, and thus only proc the 15% slow effect of the scepter. Your W's slow is much better and with your ultimate's pull most target won't be able to escape anyway. As for the health, that's all from your Rod of Ages.
Nashor's Tooth/Malady/Guinsoo's Rageblade - These items enhance her passive, I agree, but we maintained that damage with Lich Bane. There's no reason to go in a sense hybrid almost because you want to use your ball do do your damage, poking, harassing etc. The ball also keeps you out of harms way. Then again I haven't had enough games trying out these items. Will update when I tried them more =D

What about CDR?
What makes Orianna fun is having low cooldown and the blue buff. I don't purchase any items for cooldown reductions. Reason is there's the 9% from my masteries, the 10% from the Elixir of Brilliance and then there's the blue buff which if your team understands you, would give it to you. I want to spend all that gold on items enhancing my damage and defenses. What good is all this CDR with no damage?

In the beginning...
Start of by getting a Sapphire Crystal and 2 Health Potion. An alternate is to start of with the Boots and Pots instead.

Lane
Orianna is great in every lane and she can be played differently. I like to play mid because it allows me to level faster. Basically just farm farm farm. I don't suggest using your abilities to farm early game as it's more useful harassing. Position the ball beside/behind their minions. This way, you effectively increased the area you control, zoning them out easily.

First trip back...
Depending how well farmed and long you stayed in your lane, I normally pick up 1 to 2 Doran's Ring. This helps with the mana regen in your laning phase. This item is good to start off too, but the reason I don't get it as my first item is because I want the Pots to sustain myself in lane. If you have mana problems, pick up a few Mana Potion but with the Doran's Ring you probably don't need it. Therefore just pick up a few Health Potion and Sight Wards. Alternatively, you can start your build by purchasing your preferred boots and Catalyst the Protector.

As the game progresses
You will be working towards the build. The usual order of boots, Rod of Ages and then Rabadon's Deathcap might be changed depending how well you are doing. Remember you must build depending on the situation. For me, most of time I want to pick up Rod of Ages as early as possible so that it can start charging. Also this by the time I pick this up, I do enough damage to easily clear the ranged minions with a Q - E combo. After the rod, I rush the Rabadon's Deathcap so can really do some damage.

As always, remember you must consider the situation. Most of the time I would purchase my Lich Bane right after Deathcap, but there are times I might need more MR, so I get my Banshee's Veil, or more armour, making me buy Zhonya's Hourglass.

Poke poke poke! You almost adopt a Nidalee role, poking the opponent with your ball and shielding your allies when they are getting hurt. Since you have a nice AoE you want to try to pop everyone's Banshee before the team fight occurs. Remember you can throw your ball through walls, so do that.

Your role alternates between, in a sense, assassin and support. Due to your powerful range and cc, you can throw all your attack at their carries effectively disabling the opposing team. And with your shield and armour/MR bonuses, you can help your team.

Remember to protect your carries. Your E should almost be on cooldown all the time. Remember your passive for your E. Whoever is carrying ball gains bonus Armour and Magic Resist.

You need to be careful with the position of your ball as it does get hectic in team fights. Remember that if you are still in the range of your ball, and you use Command: Attack, it will actually travel the whole distance, thus potentially causing you to miss your attack or reduce the damage.

Try to save your ultimate as a CC rather than pure damage. Position your ball so you can get as many people in the area as possible, grouping everyone together. If your team has any AoE, the enemies will go down in matter of seconds =)

And that's how I play Orianna, and my little suggestions. Hope you find this helpful. Thank you for reading this guide. I'm looking forward to any feedbacks. If you downvote, please explain so I can improve and help! Thanks!