He was the seventh assistant hired by Masashi Kishimoto and worked with him on the Naruto series for nine years, beginning around volume 16.

He debuted as a mangaka with Samurai Eight: Hachimaruden as an artist, while Masashi Kishimoto is the writer. His role on the manga is to use Kishimoto's rough draft storyboards as a guide to draw the manuscript, and then ink the finalized artwork.

He has a brother called Atsushi Ookubo which is also a mangaka. His brother is the mangaka of Soul Eater and Fire Force.

Source: (Samurai 8 wikia)