YO!! This is my Orianna guide, welcome! :D
Orianna is one of my most favourite champions ever! She brings so much to the table:
High burst damage, CC in the form of a slow and a knockup, a shield (let me mention, this shields for 500 damage late game, every 6 seconds ^_~) and super high poke and vision and a speedup!!!!

If you're versing an AP mid laner, rush Chalice of Harmony. Then get Athenes, followed by Sorcerers Shoes. Then get Rabadons, Void Staff next and then Zhonyas. Your final item will usually be Liandry's Torment. The extra magic pen really helps in the late game.

If you're versing an AD mid laner, get Tear early to reduce your mana problems, then rush Zhonyas. From there build normally, just keep in mind that you won't have the bonus MR and CDR from Athene's as you'll have Archangel Staff instead.

Use Q to get any farm you think you might miss. From there, leave your ball until the enemy laner goes in to cs. When they do so, you can Q to them then W immediately afterwards for a high damage poke. Do NOT spam this early game, or you'll run out of mana very quickly.

An awesome thing about Orianna is, if you're out of mana or very low, you can still harass well with your autoattacks because of your passive.

Also, once you have a mana/mana regen item and can afford to spam abiltiies in moderation, get ready to immediately E yourself after your Q into W combo. This will negate most of the damage if the enemy laner attempts to trade back. Also, they will be moving towards you to trade, and since you now possess the ball, it makes it easier to then Q and W them again.

Teamfights as Orianna are really easy. E onto your iniator, wait for them to go in, then press R.

In teamfights you are super strong. You are a super tanky, high damage utility mage. Think about it like this. If anyone tries to damage you, you can shield yourself, negating 500 damage. If anyone DOES get you to low health, you can slow them down and run away, also speeding yourself up. If they finally get to you, you can Zhonyas!!. Then your shield will be back up again. Then, if you have Archangel's Staff, you can use that active to shield yourself again.

In teamfights you can spam Q (and W to some extent) to do constant AOE damage.

THX FOR READING ^_^
Cya :)