Onozawa made his debut for Japan against in June 2001 and immediately impressed troubling the Welsh defence throughout and scored a try from 75 metres out from a move that he started himself. From there onwards he became a fixture in the Japan side. He scored 8 tries in his first 7 caps playing at full back, but in 2003 moved to a spot on the wing. He played all of Japan's matches at the 2003 Rugby World Cup, scoring in his first match against and his wing partnership with Daisuke Ohata at the tournament was noted as one of the most positive aspects of Japan's tournament. In the 2007 Rugby World Cup, Onozawa again played in all Japan's matches and notably scored a 70-metre individual try against. Over the next few years, Onozawa gradually kept increasing his try scoring record and by 2011 he entered the top 10 on the all time international try scoring list. At the 2011 Rugby World Cup, Onozawa played in all Japan's matches for a third consecutive tournament, and also scored against to become the 15th player to score in a three World Cups. Onozawa was retained by new coach Eddie Jones, and 2012 proved to be one of his most successful years at age 34, becomingthe 5th player to score 50 international tries in May 2012, and overall scoring 10 tries in 8 matches, including tries in Japan's first ever wins away in Europe against and in November. Into his 12th year of his international career in 2013, Onozawa won his 80th cap against to become the most capped Japanese player of all time, overtaking centre Yukio Motoki's total of 79 caps. However he suffered with persistent injuries, including a long term shoulder injury which brought his international career effectively to a close with younger players coming through. He still was thought of in high regard by Eddie Jones though, who kept Onozawa with the squad in a mentoring role. At domestic level, Onozawa played for 14 years with Suntory Sungoliath, before joining Canon Eagles in 2014 to finish his career. With Suntory Sungoliath he won the Top League three times, was the league's leading try scorer in the 2009/10 and 2010/11 seasons, was named in the team of the season 8 times at the end of season awards, and was named the league's overall MVP for the 2007/08 season. In October 2012, Onozawa became the first player to score 100 tries in the Top League.