Calling it big O is indeed a mistake (especially when you start using it with handwriting, o & O because difficult to distinguish).

> Saying worst case upper bound isn't too many words and conveys the correct meaning to people with incorrect concepts about what big O means

Tricky thing is, that's not what big O says. It's a statement about the asymptotic growth of a function which you can apply to a worst case.

You'd be better off using "the worst case grows as at most". The distinction is important as you are ignoring two crucial things: the non-asymptotic behaviour and the coefficient of the asymptotic behaviour.