The simple presence of the Ball is very intimidating for champions, as they know that if they are close to it you can easily hit them. Position it to zone your lane opponent and give it a hard time farming.
You are squishy without any survivability items, but that doesn't have to be a problem - you can easily keep your distance and order your ball through walls or use it as a shield on your allies (especially tanks).
Orianna has a variety of combos according to the situation.
Since all four of Orianna's abilities can deal AoE damage and scale with ability power, she can also be built as an extremely effective burst mage by putting more emphasis on ability power and magic penetration than cooldown reduction. Builds using Sorcerer's Shoes, Void Staff, and an early Rabadon's Deathcap (as one of your first two or three items) can produce devastating results if you catch three or more enemy champions clumped up, which will happen frequently once teamfights start taking place.
Avoid over committing to Orianna or any of her allies when she's around without considerable damage. Command: Protect'sCommand: Protect's shield can be deceptively strong with the added armor and magic resistance from the Ball.