This is not a cheat sheet for playing Orianna but a guide You truly are not going to get enough out of just taking the build path and skill path as opposed to reading through this guide. As a side note, Balls OP.

Hello this is NoHats, Ap mid player of XPL, this is my first guide, and I'd like to walk you through how to play Orianna. She is a champion more based around utility, so I feel this build makes the most use of that, while still allowing her to deal (tons of) damage.

Orianna is a champ that requires a strong front-line to play to her full potential, her kit is full of poking and kiting while here peel isn't very strong towards certain champs ( Irelia and Jax we're looking at you!). That being said in lane she doesnt necessarily have bad match ups in mid lane. An example of a team with a strong front line would be Skarner, Irelia, (insert any AD here), and Leona.

As a mid laner, not just as an Orianna player, you need to be on top of your game. Here are a few tips to success with the ball tosser
-Roam, don't let your movespeed quints go to waste.
-do not overextend. Especially important for ori.
-When your lane is pushed up, ask if you can take wraiths, or even better steal enemy wraiths.
-Shield your jungler once in a while. Always a nice thing.
-Ward your lane...alot.
-Use your Q to facecheck bushes

in the mid lane Orianna isn't hard countered by any one champion, she does however, have an extraordinarily hard time dealing with silences though. Here are a few example matchups for the mid lane. In terms of base stats Karthus beats you but you have the upper hand because besides flash he has no means of escape. and its up to you to dodge his Q and shield from his ult.
Another squishy mage with no escape. The most important part of beating brand is dodging his skill shots, his Q has a cast time so use this to your advantage to try your best to dodge it and you will be fine.
FAAAAARRRRMM FEEEESSSSTTT! be careful and dodge her Q's also try not to fall behind in farm
Because nowadays most Katarina's are idiots you will usually have the upper hand. however you need to punish her stupid aggresion when posssible and remember to ward up and tell your teamates to do the same because that ***** loves to roam.
Still broken and very difficult to peel.
A good fizz typically beats a good orianna, so its up to you to be better. If not then you ask for ganks, buy health pots, build gp10, etc.

I prefer taking movespeed and health to flat AP and mana regen on ori. The reason being is that you should be stingy with your mana on ori. And on top of that, you are the ap mid. So you should be getting every blue. I also like taking movespeed on ori to help roam and because the 15 ap from flat ap runes isn't very much. The other runes are pretty Standard on ap mids.

Here I will explain my item path and a few different popular item paths for Orianna This is my favorite Item path for ori, however you should always be open to try new builds new build paths etc. And more importantly you need to build different for different situations. For example, if you are facing a team of AOE ults and you have great timimg. you can always buy Zhonya's Hourglass instead of Guardian Angel. [*] Rod of Ages vs. Rylai's Crystal Scepter and Chalice of Harmony
Rod of Ages A decent item on Orianna however I feel that Chalice of Harmony and Rylai's Crystal Scepter is a far better build path because for more skilled Orianna players. This is because it opens up more kiting potential, gives you the same AP and almost the same HP with plenty of mana regen. However it does require you be a better farmer in lane.
[*] Will of the Ancients instead of Void Staff
Will of the Ancients is a great item on ori in general, however I prefer Void Staff for the raw damage, I find that Will of the Ancients is a more situational Item, like if you find yourself getting focused down easily in team fights (easily preventable with good teammates and good positioning.) However I feel the raw damage is more suitable for [[Orianna}].
[*] Double Dorans Ring into Rabadon's Deathcap
This item path doesn't solve her mana and health problems but may be a good option if your team is lacking mid game damage and you have access to blue buff.
[*] Lich Bane instead of Void Staff A decent item for Orianna but an even better one if you are stuck within Auto-Attack range of enemies in a team fight.
[*] Athene's Unholy Grail Rush
A great item if you are truly a good Orianna player who can position well as to avoid getting hit, and your team doesn't require the ridiculous slows that Rylai's Crystal Scepter brings you.

Orianna's abilities reflect a passive aggressive play style. This is why Orianna's abilities Work very well with tanky initiators like Hecarim and Shyvana. Now here is a section of her abilities and how they should be used.(Descriptions taken from league of legends wiki. http://leagueoflegends.wikia.com/wiki/Orianna/Ability_Details)

[*] Passive:Orianna uses her ball as a focus for her spells. Orianna's ball automatically returns to her if she is too far away from it.
Orianna's basic attacks deal 10 / 18 / 26 / 34 / 42 / 50 (+0.15 per ability power) bonus magic damage. Subsequent attacks against the same target within 4 seconds deal an additional 20% magic damage. This bonus stacks up to 2 times.
This is a great tool for last hitting in lane. Use this with your other abilities to both kite and harass, an example of this is after using your Q and W on an enemy, if they begin to run chase them a bit and Auto attack them.

[*] Q:Orianna commands her ball to shoot toward a target location, dealing magic damage to targets along the way and at the target location. However, it deals 10% less damage for each unit it hits, down to a minimum of 40%.
Her ball remains behind at the target location afterwards.
Great ability for poking, I feel that Rylai's essential on her to poke and kite with this ability. Most importantly, you need to be smart about the positioning of your ball, for example if you are fighting a low health target and you know that you cant quite finish him or her off with abilities then you can Q and W behind the target in the path of escape you can Auto-Attack him down or whatever.

[*] W:Orianna commands her ball to release an electric pulse, dealing magic damage to nearby enemies.
The pulse leaves behind an energy field for 3 seconds, lowering enemy movement speed and increasing ally movement speed for 2 seconds. This effect diminishes over time.
A high cooldown ability that is excellent for clearing waves but should not be overused in lane because of its high mana cost and high cooldown. You need this ability for escaping so you need to use it wisely.

[*] E:(Passive): The ball adds bonus armor and magic resist to the allied champion it is attached to.
Bonus Armor & Magic Resist: 10 / 15 / 20 / 25 / 30
(Active): Orianna commands the ball to travel to and attach onto an allied champion, shielding them for 4 seconds. Enemies the ball passes through along the way are damaged for 75% of the shield value
Just an excellent ability, this makes it easier to face champions like karthus in lane because of the shield and bonus magic resist. This ability, in conjunction with your W, should be used to help any initiator run into battle.

[*] R: Orianna commands her ball to unleash a shockwave after a brief delay, dealing magic damage to nearby enemies and flinging them into the air a set distance in the direction of the ball.
A great ultimate for teamfighting. This ultimate has a .7 ratio and deals 300 base damage at rank 3. As I've said before, this ultimate is great when working with a strong frontline or even other AOE abilities. Seriously Try using this with a Leona ult, just awesome.

Although I don't play ranked too much, (I had some bad placement matches and I was a worse player back then :c), here are my Orianna stats as of 11/10/2012.

Remember to use Health Potion's and to buy Sight Ward's your team will <3 you. I promise!
Also don't forget about that Ashe Or Ezreal Ult. So you better recall in a safe place, you'll thank me later.

Orianna is a very versatile AP mid champion, I reccomend trying different item paths with her. She is also one of the funnest mages in the game imo. She is a champion with a skill-cap, but with time I beleive anybody can become a good Orianna player and even a good player in general. So what are you waiting for? Go out there and throw some balls!