What's wrong with "Osu!"

I've only been training Goju Ryu for a couple of months, and recently a blackbelt transferred from another dojo, and was saying "Osu!" in response to questions and following instructions. One of the other blackbelts took him to one side and said that "Osu" is banned in this dojo and has been for a few years. Apparently we should use "Hai!" in the same way. The black belt explained that "Osu" can have other meanings that are impolite or "troublesome".

I overheard this so didn't want to ask directly in that moment. But... any ideas why "Osu" might have been banned from this dojo 20+ years after it was founded?