Pronounced "Oss" (with a long "O"), Osu comes from the expression “Oshi shinobu!” which means to push oneself to the limit of our ability, and yet to continue, to persevere under pressure and to endure.  It is used as a word with many meanings, expressing the willingness to strive against all odds, to persevere on the road to physical, mental and spiritual strength, and to show respect to one's seniors as well as responsibility to one's juniors.  It is also used as an acknowledgement or affirmation or even as a greeting.