As the son of a wealthy confectioner, Okumura showed little ambition from a young age. At nineteen, he entered Kyoto University where he enjoyed the softer pleasures offered to a well-to-do college student. His father gave him 100 yen a month spending money, at a time when twenty yen was enough to enjoy a very comfortable life-style. Taking his classes lightly, he spent much time in Kyoto's dance halls along with other rich young men. Okumura left Economics Department of Kyoto University (1926) without impressive grades and was rejected by the Mitsubishi, Mitsui and Yamaguchi banks. He succeeded in entering Nomura's research department by a personal introduction.
Tsunao Okumura entered Nomura Securities Company (1926) and became acting manager of its Nagoya Branch (1932). In 1935, Okumura made a serious error which nearly cost him his career. As a research analyst in marketing, Okumura published a secret book for investors espousing the merits of purchasing overseas bonds. At the time, the Bank of Japan prohibited the purchase of overseas bonds by the Japanese. Japan needed all the money it could get to expand its military. Bank of Japan officials found out about the brochure and consequently reprimanded the then-president of Nomura securities, Otogo Kataoka. Kataoka furiously ordered Okumura's resignation but was persuaded by other senior colleagues to simply demote Okumura instead. Disgraced, Okumura was transferred to the registration section. The registration section, where only women worked, involved hand numbering every security as it was bought and sold. The only compensation for Okumura there was that the women spoiled him shamelessly. Later on, Okumura's book on purchasing overseas bonds put him back in favor with Nomura's senior management, sending his career on an upward path.
Thereafter served successively as acting director and director of the various departments of the company and became managing director (1945), senior managing director (1947) and then president (1948).
Held various additional posts including those of director of Tokyo Shokengyo Kyokai (Tokyo Security Dealers' Association) and Tokyo Securities Exchange, permanent director of Nikkeiren (Federation of Japan Employers' Associations), director of Asahi Fire & Insurance Company, member of Securities Exchange Council, auditor of Tokyo Life Insurance Company and director of Osaka Stadium.
His colleagues observed that Okumura was pleasant and self-assured but lacked the hunger that characterised other Nomura executives. He often came to work late, was despondent and left early. Shortly after joining Nomura, his parents arranged for him to marry the eldest daughter of a wealthy Osaka landowner. His marriage into wealth further discouraged his ambition.