He joined All Nippon Airways (ANA) in 1998 and was assigned to the Passenger Service Department, Haneda airport, Tokyo, where he was a check-in agent and assisted disabled people in boarding.

Starting in April 2009, he attended the ISS Astronaut Basic Training domestic program at Tsukuba Space Center in Japan.

On September 19, 2011, NASA announced that Onishi would serve as an aquanaut aboard the Aquarius underwater laboratory during the NEEMO 15 undersea exploration mission from October 17–30, 2011.

[6] In 2019, Onishi participated, with the colleagues Jeanette Epps, Joe Acaba, Alexander Gerst, Nikolai Chub and Joshua Kutryk, in the ESA CAVES[8] training organized by the European Space Agency and held between Italy and Slovenia, becoming a "cavenaut".

[9][10] In 2023, Onishi participated in the ESA PANGAEA training organized by the European Space Agency, held between Italy (Bletterbach canyon), Germany (Noerdlingen-Ries crater) and Spain (Lanzarote Island),[11][12][13] together with colleagues Thomas Pesquet and Jessica Wittner.