In Seido Karate, we use the word "osu" all the time in the dojo. To us, it means "hello", "goodbye", "I understand", "I will try", "to have patience", and almost anything else. I'm not a military man but I've seen it compared to the Marines' use of "Semper Fi" as an all-purpose greeting and exhortation. It also seems similar to the Army's use of "hooah", which has been defined as "referring to or meaning anything and everything except no".
In Japanese culture outside the dojo, it's usually considered a overly macho term. Even in some martial arts traditions, especially in koryu budo (classical arts), it's considered sort of barbaric. Karate, it ought to be remembered, was an import to mainland Japan and for many years was considered sort of "low class". Those karate people would train anyone, not just people from samurai families. How gauche!
So if you go to a kendo or aikido dojo, or even some karate dojo, people might look at you funny if you say "osu". And outside the dojo there are only a few contexts in Japanese society where it might be appropriate. But our heritage is a little more rough-and-ready. The use of "osu" was especially emphasized in the Kyokushin style of karate, the school that Kaicho Tadashi Nakamura, founder of Seido, studied for many years.
For those of us in such arts, "osu" is a very important word. So what does it really mean, literally? As is often the case in Japanese, there is more than one phrase with the same pronunciation. There is an "osu" that is short for "ohayou gozaimasu", meaning "good morning" (or more literally, "it's early!"). But we are not going around the dojo constantly saying "good morning" to each other!
Our "osu" is short for "oshi shinobu":
押 忍 お し し の ぶ o shi shi no bu
The first kanji, 押, can be read as "osu" or "oshi" (or several other ways) and means "push; attach; seize". It's a very common character, all over the place in Japan, on doors telling you to push! But its meaning to attach or to seize, i.e. to hold, might be more relevant here.
The second, 忍, "shinobu", means "to endure, to bear, to restrain oneself". The lower part of the character represents the heart, the upper part a knife. It suggests bearing up under difficulty.
Together, these character might be read "to hold to patience" or "to push through difficulty".
This idea of patience applies both to ourselves and to others. We all have desires about how we want to be, and how we want others to be. It's easy to get frustrated when we're having trouble learning something, or even when we're doing okay but dealing with a subject that takes years to learn. And sometimes we get frustrated with our fellow students, or even our teachers, when they don't act the way we want or expect them to. And those of us who are teachers can get frustrated when we realize that we're not explaining things well enough, or not properly motivating students.
So all the time, every class, we remind ourselves: Osu. Have patience. Endure.
But of course this is not just for the dojo, not just for being patient with learning a kata. It is for our whole life. Kaicho Nakamura writes in his Karate Kyohan about a student dying of leukemia:
When I went to visit him in the hospital one day, he had just had his last operation. However, there was no longer anything that could be done to save him; he was receiving oxygen, and was about to draw his last breath. The doctor called me and said he was trying to say something. I approached the bed and put my ear towards him. He suddenly took off the oxygen mask which was keeping him alive and with great effort sat up and in a voice which was hardly a voice said "Osu!" from the depths of his stomach. That voice rang out in my heart like no other Osu that I had ever heard among the thousands of times I'd heard it. He was trying that hard and had that much spirit. His breath was that important. He taught me the true meaning of Osu.
The reason that the marital arts have meaning, spiritual power, is because ultimately they are about life and death. When life presents us with a challenge, or when death -- the ultimate challenge, the knife against the heart -- arises and shows itself, what shall we do?
This is what we train for, to learn how to respond. Hold on. Be patient. Push through. Osu.