Why not to use "Oss" or "osu" - By a judo and karate black belt (anonymous)
Oss! (More properly “osu,” and even more properly not said.)
The phrase originates from Japan, and more specifically from thuggish-type martial arts practitioners. Think a feeder dojo for a yakuza clan.
The phrase might be roughly translated to “f--- yeah” (in the sense that New Yorkers say "f---"). It’s crass, rude, and completely inappropriate. Often people outside Japan who acquired the habit of saying this phrase do not convey the message that they might think it conveys.
Since kids also say "osu" to say "yeah." A lot of it is contextual, and there is no correlation in English. Japanese is a hierarchical language, where a speaker embeds his relative social standing to the recipient in the form of the words. So depending if the listener is junior, the form of some of the words will be shorter, and if the listener is senior, the form of some of the words becomes significantly longer. Look up "honorific" for a more thorough explanation.
So a senior person could say "osu" to a junior person. And a kid could say "osu," because kids are given a free pass to speak in a very casual style. But an adult student does NOT suggest that his coach is equal to or perhaps even junior to the student. It's horrifying.