In Kyokushin every question is answered with Osu. Every greeting is Osu.
Every instruction or question in class is answered by "Osu" instead of "yes" or "I understand".
When performing kihon Waza (basic techniques) in class, each technique is often accompanied with a loud "Osu". When practicing jiyu Kumite (free fighting) in class and your opponent lands a good, hard technique, you say "Osu" to acknowledge your opponent's skill. As a measure of respect, knockdown fighters at a tournament bow and say "Osu" to the front, to the referee and to each other, before and after the fight.
Osu is a combination of the words: Oshi which means "Push", and Shinobu which means "to Endure". It means patience, determination, appreciation, respect and perseverance.
Kyokushin training is very demanding. You push yourself until you think you've reached your limit. First your body wants to stop, but your mind keeps pushing you. Then your mind wants to stop, but your spirit keeps you going. You endure the pain. You persevere. That is Osu. Kyokushin karate is not learned overnight. It takes years to properly learn the fundamentals. The basic techniques are performed thousands of times (ren ma - "always polishing") until they are done by reflex or instinct, without conscious thought (mushin - "no mind"). It's easy to get frustrated by doing the same thing over and over again, especially when progress seems to be slow. To overcome that frustration and continue training takes patience and determination. That is Osu. The spirit of Osu as described by Shihan Cameron Quinn of Australia in his book The Budo Karate of Mas Oyama: "There is a saying in Japan, "Ishi no ue ni sannen." Translated, it means "Three years on a rock." This saying symbolizes the need to persevere at all times. It is one of the most important philosophies in Kyokushin karate.
Kyokushin is an art offering many things according to the immediate and long term aims of the trainee. Ultimately, one realizes that transcending the kicks, the punches, and the kata, there is a special spirit in the heart of the participants. It teaches them to face the demands of daily life with a mature and enduring attitude. A budo-ka is not easily shaken by the blows of adversity, realizing that for a person to draw near to their true potential, a never-say-die spirit of perseverance is required. This strength of character develops in hard training and is known as osu no seishin (the spirit of Osu). The word Osu comes from oshi Shinobu, which means "to persevere whilst being pushed". It implies a willingness to push oneself to the limits of endurance, to persevere under any kind of pressure.
The single word Osu captures most accurately the ultimate in what the art of karate, particularly Kyokushin, has to offer. One who is truly able to manifest the spirit of Osu in every word thought, and action may be regarded as wise and brave. Training should first and foremost be approached in the spirit of Osu.
One's daily life, and the responsibilities it holds, would be more completely lived if addressed in the spirit of Osu.
Even for the beginner, who is conscious of his lack of training and does not necessarily want to face the demand of training, it is enough merely being aware that through perseverance and the will to continue, there comes great physical, mental, spiritual, and emotional gains. All that is needed is that special determination.”