Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Orianna needs a lot of AP to do dmg that's why we max out AP items. She uses her ball to attack the enemy and that's need a lot of AP to do dmg. Follow this guide and you will do great dmg in the end.
I start out with a dorans ring and purchase Mejai's soulstealer as next item. After that i buy shoes and Rylai's Crystal scepter for some ap and hp. Then i aim for Rabadon's Deathcap. I finish my buy list with Archangel staff (selling dorans ring) and Lich Bane. Alot of AP to orianna so she can do some great dmg.
I start max Command: attack, this skill goes thru the target and when the ball hit the target it do dmg. After that i max out Command: Dissonance this u will combo with Command: attack. And it will get you good dmg. I put in ulti when you can put in ulti. Command: protect i put 1 in lvl 4 becouse it help you and your teammates with the shield u get but i max it last.
Summoner spells I use clarity and flash. Clarity becouse u get oom very easy with orianna and flash to runaway from battles. If I'm going mid i use teleport and clarity.
Follow this guide and you will do a lot of dmg in the end. Play safe and use your skills correct and you will pwn some noobs. I promise this AP build is going to make you do a lot of dmg if your mejai's get stacked. I wrote this guide fast becouse i wanted to tell you about the items and skill masteries and etc. Good Luck!