Osumi is a lawyer from Onomichi City, Hiroshima Prefecture. After graduating from Waseda University School of Law, he was a member of Tokyo Nanbu Law Office from 2014 to 2022 and currently works at Onomichi Sogo Law Office in his hometown. In addition to serving as Deputy Secretary General of the Association of Lawyers Against Nuclear Arms, he is involved in the anti-nuclear peace movement as a member of the Japan Federation of Bar Associations' Task Force on Constitutional Issues and the Hiroshima Bar Association's Peace and Constitutional Issues Committee.