2) Some Karate practicing people may be using this term in place of such as "yes sir*. However, it seems this word is a recent invention and not so much used among degnified old Karate-ka.
2) Karate-ka 1 show example, another may say "ossu".
The reason this term is used is because the bot in #GAYM will remove your ability to talk in the channel if you say "osu."
*** GAYM sets channel #GAYM mode -v Guy1
<Guy2> ossu sCK