No, that's the opposite of what parent wrote. If an engineer mentions "this makes our functions run in O(n^2)" they mean either average or worst case and actually mean theta. The interesting fact they want to express is not the upper bound but the lower bound.

I mean you say "best case O(n)" if the best case is Theta(n), even though the algorithm may be quadratic on average.

You don't say the best case is O(n^2) if the best case is Theta(n log n) even though technically that would be correct.