Born in the city of Kodaira in Tokyo,[2] Okubo started in the All Japan Road Race Championship GP125 class in 2008, riding for Endurance Honda, where he finished the season in 20th place overall with a best result of 13th at Tsukuba.

Okubo also competed in his first 125cc World Championship event as a wildcard at the Japanese round at Motegi, he qualified in 26th and finished the race just out of the points in 16th position.

Deciding to try to defend his title in 2011, Okubo stayed in the J-GP3 class, along with 18 Garage Racing Team for the third consecutive year.

He once again received a wildcard entry to the 125cc World Championship for the Japanese round held at Motegi, he qualified 25th and finished just outside the points again in 17th.

Deciding not to remain in the Japanese championship in 2012, Okubo joined the one-make Asia Dream Cup aboard a Honda CBR250 for the inaugural season.