In January 1942, Okamoto started out as a research assistant at Tokyo Women's Medical University researching the cerebellum under a neurophysiologist who "created many more opportunities for [women] than were otherwise available at the time." 
After World War II and the Second Sino-Japanese War respectively in 1945, she moved to Keio University in Shinanomachi in Tokyo. As resources were scarce, she and her husband Shosuke Okamoto changed to research on blood: "If there was not enough we could simply use our own". They hoped to find a treatment for post-partum haemorrhage, a potent drug to stop bleeding after childbirth. They began by studying epsilon-amino-caproic acid (EACA). They then studied a related chemical, 1-(aminomethyl)-cyclohexane-4-carboxylic acid (AMCHA), also known as tranexamic acid. The Okamotos found it was 27 times as powerful and thus a promising hemostatic agent and published their findings in the Keio Journal of Medicine in 1962. 
In 1966, Okamoto was granted a chair at Kobe Gakuin University. In 1980, she founded a local Committee for Projects on Thrombosis and Haemostasis with Shosuke, who also worked at Kobe. She retired from the University in 1990. After her husband died in 2004, she led the committee until 2014. She could never persuade obstetricians to trial the drug in post-partum hemorrhage.