I was going through the archived blog entries, since I'm new to the site in general, not just the forums, and I came across one topic that never seemed to have been entirely solved. (Here's the link: http://www.japanesepod101.com/2006/01/1 ... up-to-osu/ ) I apologize if this has been covered already.
"Osu" was mentioned as being used in 空手道場 as a greeting. That may be true, but that by itself is a very limited use of the word. The greeting "オッス" is an informal, colloquial shortening of "おはようございます". Take the beginning "お" and compound it with the final "す" to get "オッス". You'll hear co-workers and classmates, who are on the same level socially, use it on a normal basis.
Also, related to the topic, you'll hear "オヒッス", usually pronounced more as "ウィッス" (like We'sss), which, as you may have already guessed, means, "お久しぶりです".