Orianna is ap mage whos passive causes her autoattacks to deal more damage based on her ability power. Attack speed is always great for shoving your oppenent out of lane in the early game.
Alot of other builds I've seen tend to have the seraphs embrance, I wouldn't reccomend this item because with a grail you should have the mana to sustain any teamfight. I feel as the tear of a goddess is a waste of 700gold in the early game.
Morellonomicon is only suggested by me if you are against an Attack Damage based champion in lane. Other than that the grail seems to be the most effective item in oriannas inventory.
Plz upvote if you found my guide improved your gameplay with Orianna ^.^