OSS! You will hear it often in your journey in BJJ. It is quite common for instructors or students to use the term “OSS” (or OSU) when training or having discussions. New students who are not familiar with the martial arts culture and its traditions are usually not familiar with the term. Oss has many different meanings that are often confusing for new students to understand. I'll explain OSS!
"The famous "OSS" has many origins. The first definition of "OSS" is an abbreviation for Onegai Shimasu, which translates into a request, a solicitation, an invitation like "please", "if you may" or "With your permission", much used when inviting a partner to train.
The second definition of "OSS", also known as "ossu" Means Oshi Shinobu, which conveys the idea (the literal translation has nothing to do with the context) of "persevering when pushed" in other words, never give up, have determination, grit and withstand the most arduous of training. Carrying on without giving up, under all kinds of pressure- that's the idea of inner strength so common in martial arts culture.
Therefore, at the start of a fight, the martial artist will shout, invoke, emanate the "OSS", as a way of letting this sentiment tout. In many sources, "OSS" is defined as a manifestation of ki energy, or in other words, a different kiai(strength) indicating one is ready to fight.
According to Miyamoto Musashi, in the book "go run no who", the samurai would use three types of shouts: One before combat, the second during combat, when attacking to muster great strength and the third after the fight, to celebrate victory.
"Oss", beyond demonstrating strong spirit and determination before a fight, is also used to suggest or confirm a piece of information. Thus whenever a professor or coach of yours asks or informs something, the response is "OSS!" it is the response that will confirm understanding."