The following explanation of “osu” was taken from the U.S. Kyokushin Karate Official Text Book Basics No. 1 by [then] Saiko Shihan S. Oyama, Shihan Y. Oyama, Shihan M. Miura (Published 1981)
Osu means patience, respect and appreciation. In order to develop a strong body and strong spirit it is necessary to undergo rigorous training.
This is very demanding, because you must push yourself to what you believe to be your limit, and you want to stop; to give up. When you reach this point you must fight yourself and your weakness and you must win. To do this you must learn to persevere, but above all you must learn to be patient. This is OSU!
The reason you subject yourself to hard training is because you care about yourself, and to care about yourself is to respect yourself. This self respect evolves and expands to become respect for your instructor and fellow students. When you enter the dojo you bow and say “Osu” . This means you respect your dojo and the time you spend training there. This feeling of respect is OSU!
During training you push yourself as hard as possible because you respect yourself. When you finish you bow to your insructor and fellow students and say “Osu” once more. You do this out of appreciation. This feeling of appreciation is OSU.
Thus OSU is a very important word in Kyokushin Karate because it signifies patience, respect and appreciation. That is why we always use the word OSU; to remind ourselves of these indespensable qualities.