Osamu Oi served at Azabu Tadaemon Shoten (store owned by Tadaemon Azabu). Later opened up his own store in Yonago City. He became chief of Spots Section of Yoshikazu Shokai (Trading Firm) (1937). Founded and became president of Oi Securities Company (1947). Served also as director of Osaka Securities Exchange and Osaka Securities Exchange Association (1947-1953).