The real intent is in the way it is said.
A sloppy, lazy, "oossuu" is like the shortened "Ohayou gozaimasu" or "Onegai Shimasu" - disrespectful.
A strong "OSU" with eyes bright and snilimg is very respectful and a joy to an ubderstanding Sensei.
A hissed "Osu" with a black look is similar to the sloppy, lazy way, but with bad feeling.
If you ask any Military Officer, they will explain this in the same way as the troops can say "Yes Sir!"