It is said that he succeeded to Ogasawarago, Koma district, Kai Province among the territory of his father Tomitsu Kagami who served Emperor Takakura as an Imperial Palace Guard and that he was granted the family name of Ogasawara from Emperor Takakura when the time came to celebrate his coming of age.
高倉天皇に滝口武者として仕えた父・加賀美遠光の所領のうち、甲斐国巨摩郡小笠原郷を相続して、元服の折に高倉天皇より小笠原の姓を賜ったとされる。