Hello and welcome to my first guide! I have found Orianna to be a great champ, and one of the more useful all around champs when played properly. Now, this guide wont make you go 30-0-30 without even trying, you have to learn how to use her properly and all the strengths and weaknesses of Orianna. So, with all that said, let's get started!

Orianna is a very flexible character, having the ability to fill lots of roles needed on a team. The role I find her most useful as is a chaser/killer; killing people who are low on health in team fights and chasing runaways. She has good damage which means she can carry a team, but also has a shield which is very very helpful in supporting your team mates. She is a true team player in my opinion: offering everything but tanking and melee dps to a team.

Pros
Great harasser
VERY STRONG MID GAME
Great utility and team fighter
Doesn't need to be fed in order to do well
Not super squishy like an Ashe
Has problems that can be solved via items, runes, and masteries

Cons
Very mana dependent
Low base stats for health, armor, and magic resist
Cool down on spells is a bit long to be extremely useful
Takes time to learn all the mechanics- not an easy champ to start with

Clarity

One of Oriannas major problems is her low mana pool. A manaless Orianna is a dead Orianna. So, my answer to that is Clarity! One click of a button and BAM you're at half mana or more. This is my staple for her and almost say it's a necessity for her.
Other Viable Summoner Spells

The second summoner spell I've fiddled around with and haven't found one I absolutely love yet, so here's a list of ones I've used and found to be good with her.

Heal
Heal is useful to me because she is kind of squishy as all casters are, and have that little health boost gives her more sustainability in lane and can give her enough health to get out of trouble. Pretty simple.

Flash
Great for getting out of a sticky situation. I switch between this and heal the most. Good to close distance as well for a kill.

Ignite
Offensive use is great for not letting that kill barely slip away, but hasn't shown me a lot of use late game.

Ghost
Speed never hurts, but you already have a slow/speed boost in dissonance. Not too useful but can have its purpose

Clairvoyance
Useful tool for brush checking and map awareness, not for everyone but some people love it

The other summoner skills I'd avoid due to their lack of usefullness with Orianna

I got offensive just because I like to pick up those early kills, but keep some defense because she is a caster, and is semi squishy. Of course if you are getting killed a lot, switch to a 9-21-0 Masteries set up.

THIS IS NEW! I tried a 0-9-21 build and it helped with the early mana problems! I really liked it and found it to be very useful

Marks

I grab magic pen because there's no other really good mark for casters, plus it makes her auto attack combined with her passive a little more useful.

Seals

Once again Orianna's biggest problem is her mana pool, so give her some mana!

Glyphs

Orianna's spells have a longer cooldown than I'd like naturally. So, add some cool down reduction glyphs along with the item build and that problem is solved

Quints

Orianna has a great ap scale, and with my build Orianna doesn't really get any ap until mid game. So, this gives her a small ap boost early on, which in turn helps with laning and such.

With Orianna, you use a basic combo of Attack and Dissonance. It's your 1-2 punch that is key to playing Orianna well. With that said, I don't max out one before the other! It's a weird concept for a lot of people. This way if you miss a champ barely with Attack (and you maxed it out first) but hit them with Dissonance, you won't do any serious damage to them without keeping equal stats on both. If you HAVE to max out one before the other, max out Dissonance. It has a better damage output and slow.

First Purchase

At the start of the game I pick up two Faerie Charms and three health pots. The two charms help your mana problems early on and build into Philosopher's Stones, a crucial crucial part to my build. Now, I use to pick up the two Charms and no health pots, but found the health pots to be vital. YOU NEED TO BUY THE HEALTH POTS!!!!! They allow you to stay in lane long enough to have enough gold to buy both Philosopher's Stones at one time, which is very nice instead of having to take two separate trips and sacrifice exp during that time.

Second Purchase

Two Philosopher's Stones. Why two? Because you need gold, mana regen, and health regen. It allows you spam your skills longer and make you less squish due to health regen. The gold is the icing on the cake. The build is a tad expensive, so getting that extra gold will give you an item advantage mid game, which allows you to dominate mid game.

Third Purchase

The catalyst and boots. Orianna has a cooldown problem, so problem solved! Now you can really spam her abilities and make other people's lives miserable. The catalyst is a great item as it makes you once again less squishy and gives you mana. This will build into the Rod of ages into your next purchase.

Fourth Purchase

Buy Rod of Ages, this item should be an Orianna staple. It gives heath, mana, and ap! That allows you to now pick up some kills with that ap boost. Also, if you have enough extra gold, build the Sheen. This once again allows you to be stronger, plus to passive give your auto attack a little extra power, never a bad thing.

Fifth Purchase

Linch Bane. This item makes you scary as Orianna. I mean scary. Move speed boost to chase down kills, ap, and enough mana to finally get rid of that mana problem entirely! The passive auto attack boost is nice, but I don't notice it as much as the other three stats given by linch bane.

Sixth Purchase

Deathcap give you ton of ap. Being an ap caster, you love this item! It increases your damage so much and boosts your shield a ton! Also if you have enough extra gold go ahead and sell one of your Stones to buy a Will of the Ancients. Spell vamp plus a team boost in ap will make everyone on your team love you :)

Final Purchase

Sell the last stone and pick up a defensive item. I almost always go Guardian Angel because I don't die much with Orianna, so for that one time I do, I'm brought back to life and can run away. Banshees Veil works well here as well.

Other Viable items

Like any good player, you build to fit your situation. The items listed above are my usual route, but sometimes will change up the items to fit different needs. Besides, who doesn't like options?

Morello's Evil Tome
Mana regen, ap, and cdr. What's not to love?

Shurelya's Reverie
Possibility instead of Guardian Angel, Health, free ghost, and cdr

Banshees Veil
Useful item if one person is trying to focus you down and stun you. Solid defensive item.

Eliesa's Miracle
Tenacity for if the other team has lots of crowd control on it. Plus its an upgrade on the Stone so it makes the build a bit less expensive.

Void Staff
Little magic pen never hurt anyone

Zyhonas Hourglass
This used to be a core item in my build. Useful ap plus 50 armor helps the squishy problem.

Now that all the extraneous stuff has been covered, let's get to the actual game play! Early on you're main job is to harass the other champs and farm minions. I use Attack to push the ball towards the enemy champ, Dissonance to farm and slow IMMEDIATELY after the ball has stopped moving. That is your main spell combo and you will SPAM that combo almost the whole game. Usually around level three I run out of mana, and pop clarity at this point, allowing me to stay in lane with a purpose. I cannot stress how important it is to spam that combo to get as many minion kills and harassment as possible. If you see yourself getting low on health, fall back for a second, pop a health pot, and get back to work. You need to stay in lane as long as possible in order to make as few trips back to base as possible. Orianna is weakest between levels 1 and 4, so protect yourself in between those levels and make it to mid game. Once you've gotten your two stones and boots it's about time for mid game.

This is when Orianna really shines. Usually I pick up anywhere from 4 to 10 kills in this time period. Once you start getting your ap, if you're still laning go ahead and play aggressive, but not over aggressive. go through your combo as much as possible on a champ, and where they're about to run away to safety, use your ult. Now, Orianna's ult is kinda like urgot's or janna's by moving people around, her ball acts kind of like a magnet, and flips their position onto the other side of the ball. How is this useful to you? You use this to bring people back into range, I've found after using this I can go through the combo at least one more time, and pick up the kill. Also, look for ganks! Orianna synergizes very well with just about any champ, and is great for helping in ganks. If a team fight breaks out, your job changes drastically. You now use your shield to protect people alot more, and damage people ocassionally. Usually I protect team mates and then when I see a champ getting low on heath, focus on killing that champ. Orianna is great at picking people off due to her damage output with Attack and Dissonance, plus the slow makes it almost a guarantee kill. You NEVER EVER EVER get in the middle of a team fight! You are squishy and will get wrecked if you do so. If you are getting focused on, immediately cast Protect and then Dissonance, this will give you a shield, armor/magic resist boost, plus a move speed boost and slow them down. With all of that, you should get away and they give up.

Late game is alot like mid, but people now have their defensive items and you are on tier with everyone else. Keep harassing and Protecting in team fights, farm whenever possible (she is an amazing farmer with her basic combo) and win some games!

So, if you don't feel like reading all of that and understanding it all, here's the basic idea of her:

Harass Harass Harass
The basic combo is Attack, then Dissonance
Use Protect often in team fights, your team mates will love you
Use your ult to secure kills and bring them closer to team mates/yourself
Thrive mid game, survive early game
Stay on the outside of team fights
Have Fun! :)