Okubo began racing in the All Japan Road Race Championship GP125 class in 2008, riding for Endurance Honda. He showed steady improvement through his early years, finishing sixth overall in the GP125 class in 2009 with the 18 Garage Racing Team. In 2010, the class was renamed J-GP3 and Okubo delivered a breakout season, securing victories at Tsukuba and Motegi along with two additional podiums to claim his first Japanese championship title. He also made his first 125cc World Championship appearance as a wildcard at the Japanese round at Motegi that year, qualifying 26th and finishing 16th.
Okubo attempted to defend his J-GP3 title in 2011 but managed only tenth overall after completing just two of five races. He again appeared as a wildcard at the Japanese Grand Prix, qualifying 25th and finishing 17th.
In 2012, Okubo stepped away from the Japanese championship to contest the inaugural Asia Dream Cup, a one-make series aboard the Honda CBR250 that ran alongside the Asia Road Racing Championship across six Asian countries. He won the series title in a close battle with fellow Japanese rider Hiroki Ono, prevailing by ten points.
Okubo returned to the All Japan Road Race J-GP3 class in 2013 with the Hot Racing Honda squad, finishing third overall. He remained in J-GP3 with Hot Racing in 2014, placing sixth in the championship with a best result of second at Motegi. He also received a wildcard entry to the 2014 Moto3 World Championship round at Motegi but did not finish after qualifying 26th.
For 2015, Okubo moved up to the ST600 series with Kohara Racing on a stock Honda CBR600RR, finishing third overall in the Japanese championship with podiums at Okayama and Suzuka to close the season.
In November 2015, Okubo signed with CIA Landlord Insurance Honda to compete in the 2016 Supersport World Championship, his first full international season. Aboard a Honda CBR600RR, he scored points in six of twelve races and finished the championship 21st overall, with a best result of tenth at EuroSpeedway Lausitz.
He remained in WorldSSP for 2017 with the same Honda setup, improving to 15th in the championship standings. His standout result that year was a sixth-place finish at Buriram International Circuit in Thailand. In 2018, Okubo joined the Puccetti Racing squad and switched to a Kawasaki ZX-6R, finishing 13th in the championship with a best result of eighth at Circuito San Juan Villicum in Argentina. He continued with Kawasaki Puccetti Racing into the 2019 season.
In February 2021, Okubo was announced as a competitor in the 2021 MotoE World Cup with the Avant Ajo MotoE team, replacing Niki Tuuli. The MotoE World Cup, run as a support series to MotoGP, uses the Energica Ego Corsa electric motorcycle. Okubo later competed in the MotoE World Championship with the Tech3 E-Racing team.
Hikari Okubo represents a generation of Japanese riders who navigated both domestic and international championships, working up through the structured ladder of Japanese club racing before finding sustained competition in the Supersport World Championship. His involvement in MotoE also placed him among the pioneers of electric motorcycle racing at the world championship level.