btilly's comment is exactly correct (though obviously not a complete explanation of everything).
The S.O. link you posted contains an Accepted Answer which is also exactly correct, and happens to agree exactly with btilly.
I don't know what you think you're saying, or what you think that SO link supports. But it is NOT the case that "big O is worst-case", nor is it the case that the variants like little-o/big-theta/big-omega/etc have anything to do with best/worst/average case. The relevant sense of "upper bound" is not some kind of subtle synonym for "worst case".
The S.O. link you posted contains an Accepted Answer which is also exactly correct, and happens to agree exactly with btilly.
I don't know what you think you're saying, or what you think that SO link supports. But it is NOT the case that "big O is worst-case", nor is it the case that the variants like little-o/big-theta/big-omega/etc have anything to do with best/worst/average case. The relevant sense of "upper bound" is not some kind of subtle synonym for "worst case".