Osu is a combination of words: Oshi, which means «push», and Shinobu, which means «endure». Osu means patience, determination, gratitude, respect and perseverance.
To develop a strong body and a strong spirit, you need to go through complex training. Kyokushin training is very demanding. You become demanding of yourself until you decide that you have reached your limit. At first your body wants to stop, to give up, but your mind keeps pushing you. Then your mind wants to stop, but your spirit supports you. When you reach this point, you have to fight yourself and your weakness, and you have to win. To do this, you must endure pain, you must learn to stand your ground, but above all, you must learn to be patient. This is Osu!
Kyokushin karate is not immediately learned. It takes years to learn the basics properly. Basic techniques are performed thousands of times (Ren Ma —«constant polishing») until they are performed by reflex or instinct, without conscious thinking (Mushin — «no mind»). Don't get frustrated by doing the same thing over and over again, especially when progress seems slow. To overcome this frustration and continue learning, you need patience and determination. This is Osu.
The reason you put yourself through hard training is that you take care of yourself, and taking care of yourself means respecting yourself. Self-respect needs to be developed so that it spreads as a respect for the Sensei and fellow students. When you enter the Dojo, you bow and say «Osu». This means that you respect your Dojo and the time you spend there. This sense of respect is Osu!
Thus, Osu is a very important word in Kyokushin karate because it means patience, respect and value. That is why we always use the word «Osu» to remind ourselves of these essential qualities. Real faithfulness needed in order to follow the way of Kyokushin is Osu!