I try not to. People say that it is generally not a polite phrase in Japanese Society. Also, it's way over used here in the West. People use it in place of the word 'hai', meaning 'yes' or 'I understand' or they use it for any general acknowledgement. They also do it when they bow to each other. Sometimes people do it when they bow at the beginning of their kata to indicate spirit. It so saturates the tradition that after I am done training with the person and I bow I sometimes say osu accidentally, but I try not to. I guess I also noticed my students do not unless they come from somewhere else. I think the younger generation in Japan is also using it, but it really used to bother some of the Old Masters, so I think we should discourage it.

I'm actually thinking on the same waves as you here, I remember way back when that word in our class was used so much and like you said , in place of yes or no . Thank you for the reply!