Thanks for the welcome, kirakira...if you want to use it with your Japanese teacher, make sure not to voice the 'u' sound. It's devoiced, meaning that instead of pronouncing two distinct syllables "oh-suu," you would pronounce it as only one, "ohs."
hahaha, and if you wanna get really authentic, you can make 2 fists and cross your arms at the wrist in front of you, parallel to their(the person you're speaking to at the time) body, and then uncross them and pretend to beat two invisible drums in front of you at waistlevel(beat the drums only once please), and THEN say "osu".
Now that would be the true karate way.
So hypermasculine...i love it hahaha