The strength of this build relies on good farming tactics. If you are not someone who can perform the last hit as much as you need to, this may be a good build for you. It is not in depth. Much of Oriana's playing style must be learned through playing her and not reading about her. Simply put, each ability has a different focus on it. Q-emphasizes harass and continuous damage. W-emphasizes excellent farming, damage output against champs, and crowd control. E-emphasizes survivability, and utility. R-Its an ult, need I say more.
******The largest difference in many other builds and this one is the ORDER OF ABILITY CHOICE. Regularly I would agree that you should level up your Q before anything else. However, sometimes you fight an opponant who has resist, or is just plain healthy, and they can zone you. So instead of wasting tons of mana on harass to get close to minions, why not just farm minions without that enemy champion being able to touch you. I recommend that you throw ball to the MAGICIAN MINIONS because they are weaker and are able to be killed easier. Then you use your AoE/W to kill them. This only will work once your W is leveled up to 4, or 5. Most of the time your AP determines when you can kill minions, but when your W hits level 5, AP is not an issue. A simple Q--->W combo will kill the magician minions and farming becomes incredibly easy. With good farm, the process of getting items and turning into an AP carry becomes an easy task.