Could not follow those links to Osu given above. Here's my take. I trained in a University dojo in Japan for one year and what I noticed was this:
1st years - most polite: They said, "Onegaeshimas" and "Domo arigatou gozaimashita."
2nd - fairly polite: They said, ... less sylables
3rd - so so polite: They said, ... even less syllables
4th - most brusk - They said, "Ooss" and "Ooss"
Seems to me "Ooss" is just being part of the in crowd, or perhaps, just plain lazy!