He is famous for the Gell-Mann–Okubo mass formula for mesons and baryons in the quark model; this formula correctly predicts the relations of masses of the members of SU(3) multiplets in terms of hypercharge and isotopic spin.
[1] Ōkubo began study at the University of Tokyo in 1949 and received his bachelor's degree there in 1952.
He became a graduate student at the University of Rochester in 1954, where he earned his PhD in 1958 with David Feldman as thesis advisor.
In 2005 he received the Sakurai Prize from the American Physical Society; "For groundbreaking investigations into the pattern of hadronic masses and decay rates, which provided essential clues into the development of the quark model, and for demonstrating that CP violation permits partial decay rate asymmetries".
[4] In 1976 Ōkubo received the Nishina Memorial Prize in Japan and in 2006 the Wigner Medal.