There are such conventions. Take a look at an algorithms textbook, or a theory paper. That some people are semantically lazy does not change these terms have established definitions within computer science.
Well, the idea of conventions is that people agree to use them. I see a lot of variation in the degree to which people know about or are attached to this one.
While there may be a set of textbooks and papers for which the editors would have flagged "worst cast O(..)" as redundant, I think it's relevant to point out that the use of big-oh notation in mathematics predates computer science. So to the extent algorithm analysis is a branch of mathematics, those who use big-oh in this more general way are in fact consistent with the larger body of work.
Those are the conventions in Computer Science, but the conventions in Software Engineering are different. Words having different meanings in the jargons of related fields is a really old problem, and one which has bitten me from time to time. At least we aren't arguing over whether a factor of 10 increase in something is a 10 dB or a 20 dB change.
