First of all you must know orianna is relly hard to play champion, shes only power is passive and ball, because of this you must have large cooldown reduction to can move ball as fast as you can, becayse of moving ball you will lost lot of mana so you must have items who will give it back.
So I'll sey some think about skills now...

Because of less cooldown per level of skill you must have yous Q ability maxed as fast as you can. Secound skill is your W who deals main dmg, and getting you able to run awey for enemies or cach them when they running awey so ids myst be maxed ad lvl 13. E abilit is a support and def skill who giving you passive armor and magic rezist when you have ball, or giving it to your ally when he have it. R skill(ulti) like alweys at lvl 6,11 and 16 this ulti get enemy stun and throw it to you or awey from you and its relly help full on team fights so dont use it on single enemy.
So I have sed everythink about skills now it's time for items...

Early Game:
- two dorans ring gives to you some HP, AP and mana regen and they are not too expensive.
- Ionian boots giving to you fast cooldown reduction so this is relly important.
Mid Game:
- Unholly grale giving to you what you need so it's mana regen at relly high level and giving to you some cooldown reduction and of corse AP.
- Rabadon is normal AP item so I don't need to say about it.
- Maldady you need to have AS to passive and becayse of uniq sikll of it who gives you more dmg form auto attack and meaking enemy more able to get dmg from your skills.
Leate Game:
- Lich Bane is item who meake you to deal some think like 1000 dmg from auto attack after skills and it's gives to you some MS, AP and mana.
- Nashor's tooth giving to you agein some AS to passive, and its gives to you some more cooldown reduction and AP of corse.

So now when I have end with items we will pass to runes...

This runes what I preforme giving some leate dmg and helping to win on mid line on early game. Flat Ap quintesencies you need on early and another thinks you need on leate game.
So now I'll sey some words about masteries...

It's normel Ap masteries only change is two points to AS, who gives help to your passive.
After this short chapter we will come to farming...

Farming with orianna it's relly eazy think because of passive and this: on the 9lvl you killing magic minions by Q+W so with no poblems you can have 150 LH on 20:00.
So now we will see a summary...

I know this build don't looks as another builds but if you try to meake build you will see this not to eazy so don't sey bad words about this byld because of looks. I wos only trying to help you with play with cham who I relly like and I give you my best build to orianna and I thin if you try play with my build you will do sole nice dmg for example my team won a game when team stats wos 33/60 only because enemy can't kill me with this build :), so see you soon on League of Legends.