to clarify: if we're talking about worst case, we should use big O, and average case can be big O, but best case I should use Big omega and not big O since it means we can do better which isn't true (since big O classifies the upper bound) and Big omega classifies some c s.t. cg(n) is same or belwo some f(n) so cg(n) is the lower bound, so we can do as good or worse. It's just that wikipedia uses big O for worst case, best case and average cases.
e.g. for quick sort it be:
average case/best case: Big-Omega of nLogbase2n (when list is half sorted)
worst case: Big-O of n^2 (when items are sorted or reversed