I have exhaustively combed websites and trawled message boards across the entire internet to bring you the most comprehensive guide to the meaning of Oss yet witnessed by mere mortals.
What is clear to me from my years of study of the term and use of “Oss" in Brazilian jiu-jitsu and other, inferior forms of martial art is that Oss stems from Okinawan karate. Derived from “Osu”, it’s important to understand that the term is about as versatile as it comes. Oss has several uses ranging from a greeting to answering a question to acknowledging an opponent’s skill and as a motivating call to action. In fact, the term is really really really versatile.
The earliest definition of “Oss” was used as as an abbrevition of a translation of a sparring or training request, like “please/with permission let's roll".
It can be used as a greeting some schools use “Oss” with a simultaneous shaka as a form of greeting. Some places employ it as a standard salutation for students when entering or leaving the dojo.
Oss can be used in response to a question in the dojo, and the term is also used in place of “I understand” or "I've got it" during technique demonstration.
High belts may use Oss as a way of showing approval for a particularly hearty effort from a lower ranking belt in sparring.
Oss is used as a show of respect when at tournaments. It can be heard while bowing to your referee and opponent before and after a comp clash.
Oss is also used as a compliment when acknowledging the skill of an opponent or training partner during the execution of a technique or good performance in a roll.
Oss is also used in some schools during the perfomance of basic techniques. The term is reportedly said as each technique is carried out.
A student may also use the term to assure their professor or instructor that they are committed to continue despite being tired. This may well be the highest form of Oss because it demonstrates character and the will to train hard.
Some have likened Oss to the military's use of the term “Oorah”. Often used as a war cry by marines but also as a call sign or acknowledgment when off duty amongst civilians.
Some people will use Oss in their everyday life even just answering normal questions. Those people may see Oss as a specific mentality and bring it into their way of life.
Oss can also be safely employed when there is nothing else left to say. For example "I have to leave now see you next time". Your partner may reply "Oss".
Beyond this Oss may be used in BJJ schools for innumerable reasons. Since the onset of social media and group chat the use of Oss has broadened considerably extending beyond an expression of will or determination and is now also used in response to a particularly impressive or funny meme.
Broadly speaking there aren't many scenarios in the dojo that Oss can't be reasonably deployed. Now you hopefully understand the meaning of Oss you can dispatch it with confidence. Comment below if I have missed any of the nuances in the deployment of Oss.