Oshi was one of two rōnin (Sato being the other), who Raphael trained in ninjutsu to kill time while Renet and the turtles were in feudal Japan. They also accompanied the Turtles and Renet on their siege on Savanti Romero.
After their time-traveling friends left, Sato and Oshi decided to apply what they'd learned from Raphael and start a clan of their own. Since every journey starts with a single step, they decided to call this clan The Foot.