On the meaning of "osu":
I do hear it occasionally, mostly from guys behind the bar or counter at the udon shop. Even as a toast, "cheers", "kampai".
Most interesting, though, was a scene on a TV show I saw my first week here. Before my cable got hooked up, all that was on TV was Japanese broadcast channels, and sometimes I'd watch, try to figure out what I could.
One show was some sort of medical drama, and part of the plot seemed to revolve around a young female doctor trying to deal with a difficult patient. In one scene, she'd apparently made up her mind to confront him or tell him something. And as she set off down the hall, she took a moment to gather her courage and determination, and said to herself, "osu!"