In 1946, Mas Oyama went into training at a remote spot on the Mt. Kiyosumi in Chiba prefecture. He was accompanied by one of his students named Yashiro. A friend, Mr. Kayama brought them food supplies every month. Through vigorous training, Oyama learned to overcome the mental strain caused by solitude but Yashiro could not bear it and fled after 6 months. About fourteen months later, Mr. Kayama told Oyama that due to unforeseen circumstances he could no longer sponsor Mas Oyama's retreat in the mountains and so Mas Oyama's original plan of remaining in solitude for three years was brought to an end. He came out of seclusion and began teaching his own style of Karate, Kyokushin-kai, around the world including the United States. Oyama Karate became known throughout the world for its tremendous power. In 1950, Mas Oyama began his famous battles with bulls, partly to test his strength and also to make the world sit up and notice the power of his karate. All together, Oyama fought 52 bulls, killing 3 instantly and taking the horns of 49 with knife-hand blows.
Mas Oyama opened his first "dojo" the Kyokusin Kaikan in 1953 in Mejiro, Tokyo. This was the time that Mas Oyama's karate strength was at its peak so the training was severe. Many students were members of other styles and Mas Oyama would compare styles and build on his karate. He would take what he felt were the best techniques and concepts from any martial art and gradually fit them into his training; therefore, laying the foundations of Kyokushin "ultimate truth" karate.