We said the same thing in Burmese martial arts classes in Australia, and also while doing kickboxing in Amsterdam (although in both cases, it was said as "oos" instead of "oss"). It may be a martial arts thing in general, although I don't have the experience to confirm that.
It just means "Yes" or "I understand". For example, the instructor might say "Do 50 pushes", to which you'd say "Oos" right before doing it. I have no idea where it comes from - maybe a shortened version of "yes"?