Orianna is a quite difficult champion to utilize if you aren't aware of where your ball is. Make sure you always see your ball and you do not constantly walk away to make the ball auto return at bad times. (I swear I accidently used dissonance to myself like 100 times when I first got her. Also make sure you are either always harassing the other team since you are in a safe range while your ball does all the damage. She is also a great solo mid character because of her early harassment.

I have completely changed my build for this.
The first few items is just for survivalbility in early game
I rush my Rod of Ages just so I'm not as squishy later in the the game and it's passive helps too.
The Malady is a personal favorite because it gives your auto attack an extra 20 AP damage which stacks with her passive.
Lich Bane is also extremely nice to go with her passive. It does 100% of AP with my auto attack which just makes her stronger and stronger. + that extra speed and m.resist helps a lot too.
Next is my arch angel's. If you haven't noticed most of my items are filled with mana. So get an arch angel's and all of that will go to your AP. + you'll never have to go back to town just because your out of mana. You're pretty much on an endless supply of mana
Deathcap is just an obvious choice. All your AP + 155 + 30% of that is just deadly.

Obviously you want to max her Q spell as quickly as possible because it makes her a great source of damage and easy farm.
Her W spell should be maxed second because of how much help it can do for your team. It slows down enemies which help your team catch up or speed up your team to get your team to run away. it also does an awesome amount of damage.
Her E spell can be extremely effective in certain situations. I can't tell you how many times my ball got a kill just because of was returning the ball back to protect me. Also when your initiating teammate goes in, make sure you E spell on them. It will first live a long duration and since the enemy is surrounding the initiator, you can use your spells from a far distance and take them all out because your teammate has the ball.

My favorite combo is
Q,W,R because you damage and then damage+slow then damage+fling+they are still slowed. pretty much... the team is gone with the help of your teammates.
With the initiator though....
It's a E,W,Q,R
For reasons I already explained. I put that Q in there just in case they are not in range for my ult.