He graduated from Seiko High School in Yokohama in 1994 and received a Bachelor of Engineering degree in Aeronautical and Space Engineering from the University of Tokyo in 1998. Onishi completed two years of basic flight training in Bakersfield, California and one year of advanced flight training in Tokyo.
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. He was promoted to co-pilot of Boeing 767 airplanes in October 2003 and flew both domestic and international routes. In February 2009, Onishi was selected by the Japanese Aerospace Exploration Agency (JAXA) as one of the Japanese astronaut candidates for the International Space Station (ISS).
Starting in April 2009, he attended the ISS Astronaut Basic Training domestic program at Tsukuba Space Center in Japan.
Onishi arrived at National Aeronautics and Space Administration"s Johnson Space Center in Houston, Texas, in August 2009. Once Onishi is trained as an astronaut, he will have the opportunity to fly to the International Space Station and engage in scientific experiments at Japanese Experiment Module space laboratory, as well as in the maintenance and operations of the station.
On September 19, 2011, National Aeronautics and Space Administration 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. Delayed by stormy weather and high seas, the mission began on October 20, 2011.
On the afternoon of October 21, Onishi and his crewmates officially became aquanauts, having spent over 24 hours underwater.
NEEMO 15 ended early on October 26 due to the approach of Hurricane Rina.
As one of the 14 members of the 20th National Aeronautics and Space Administration astronaut class, he is currently participating in Astronaut Candidate Training which includes scientific and technical briefings, intensive instruction in ISS systems, Extravehicular Activity (EVA), robotics, physiological training, T-38 flight training, and water and wilderness survival training.