When you look at the characters that form the word, the first character means to push, to control, to surpress and symbolizes 100% effort. The second character has a picture of a heart and means to endure, or to conceal, it can also mean to be silent.
Together Osu! is a pledge to do ones very best and to endure.
Karate involves a great deal of self reflection.
A karateka that devotes energy to attracting attention rather than to learning karate does not act in silence.
The effort should be to learn karate, not to impress you teacher.
A karateka like Sensei Trevor has always been humble and reserved. His efforts in self training where in the main independent of recognition of others, he is indeed our Sensei.
It is difficult to continually challenge yourself, to work hard even though there are no others to provide encouragement. To improve we all need to work on our fitness, flexibility, Kihon, Kumite and Kata outside of our dojo training.
Remember each time we say Osu! we are pledging to work hard.