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Oki began studying koto as a child, studying under his mother, who was a professional kotoist. He took up trumpet from 1955 and played in high school bands, then enrolled at Osaka Industrial University, where he majored in architecture and concurrently played in Dixieland jazz ensembles.
Early in his career, Oki studied under Fumio Nanri, Kenny Dorham, and Sadao Watanabe, and in the 1960s and 1970s played with Nobusuke Miyamoto, Yoku Tamura, Kosuke Mine, and Akio Nishimura. In 1966, he was a cofounder of ESSG, along with Masahiko Sato, Mototeru Takagi, and Masahiko Togashi.
In 1974, Oki relocated to Paris, where he played with Japanese expat Takashi Kako and played across Europe with Art Farmer, Maynard Ferguson, Noah Howard, Lee Konitz, Steve Lacy, Michel Pilz, Sam Rivers, and Thangodeï. In 1992 he became a member of the World Residents ensemble.