I recently received an email concerning this subject and decided that I would try to give an overview of the expression. This was previously covered a while back before the Warrior Pages moved to ning and was lost somewhere in the transition. (This is also a discussion in the Japanese Language Forum)
Osu is one of those Japanese expressions which Americans do not fully understand and I have personally noted it's over-usage in several instances.
Americans are very stubborn on occasion - especially if they think something is correct because that's the way they've been taught. After some research (which I encourage everyone to do to confirm anything you are unsure of...) you may actually learn that you have been doing it wrong for a L-O-N-G time.
I could discuss this extensively but I ran across a great article by Rob Redmond on the correct usage of Osu a couple of years ago and it was very complete - it is also very long - but well worth the read.
If "Osu" is an expression that you use in your dojo - you should definitely read this article so that you will educate yourself about the meaning of the expression and not misuse the expression around other Japanese martial artists. You might just save youself some unintentional embarrassment.
Trying to use an expression in another language sometimes does not confer what you might think - such is the case with the term "Osu".
"Appropriate Usage of Osu"
There is another article by Rob Redmond on the same subject which covers -
"Do you say Osu too much? The Expirit de Corps of Osu! or Maybe Not"
I strongly suggest you read both articles and then provide some comments. It may change your perspective. I certainly do not encourage any of my students to use the term unless they can tell me the uses of the term so that I know they understand it.
Enjoy the read - I'm anxious to hear back some comments.