Oshi Shinobu

Osu is short for “oshi shinobu”. Osu is two words shortened/compressed into one.
Oshi = push; pressure. Shinobu=persevere; endure
So the meaning of osu is roughly to endure under pressure. To never give up.

Osu no seishin is the kyokushin motto and consist of two words. Osu and seishin.