So this meme that Orianna has a huge amount of burst damage needs to die off.
Her ult in particular is given incredible amounts of hate for how much damage it does.
So how much damage does it do?
300+75% AP magic damage.
That's 60 more damage than a Xerath Q. A single Annie W will do more since it only has 50 less base damage but an extra 10% scaling damage, meaning she only needs 500 AP to break even with Ori's ultimate.
So how much does Ori's full combo do?
1,010 base damage (I'm including the ball flying through enemies for her E and a luden's proc) + 230% AP magic damage.
Let's compare her to an AP assassin like Fizz. Just for fun, I'll assume a minimum range ultimate and a worst case usage of his W, and not factoring in his W's bleed since it isn't burst. I am factoring in lichbane however.
832 + 280% AP magic damage.
So Fizz has a bit less than 200 base damage, but 50% more scaling damage.
What does this total out to?

This is a generic Ori build. Plus the 30 AP she gets from runes and masteries, this totals out to 635 AP. So her scaling damage would be 1,459.3.

This totals out to 594 AP. So his scaling damage would be 1,663.2.
So Ori's total damage is 2,469.3, whereas Fizz's is 2,495.2. Which is to say, a Fizz that doesn't get the full damage of his ultimate or his W will still do more damage in a full rotation than Orianna. A Fizz who lands a max range ultimate and procs his empowered W will do considerably more damage, likely by over a thousand.
No, Ori's main damage power late game comes from her heavy sustained damage, where she abuses the incredibly low cooldown on her Q to kite around and chunk people over and over again. But her actual burst potential? It's not very high unless she's snowballing.