Okubo was born in Kanda, Fukuoka on June 9, 1982. After graduating from Kunimi High School, he joined J1 League club Cerezo Osaka in 2001. He played many matches as forward and offensive midfielder from first season. However Cerezo finished at the bottom place in 2001 season and was relegated to J2 League. In 2002 season, he scored 18 goals which was the 2nd top scorer and Cerezo was returned to J1 in a year. From 2003, he scored more than 15 goals in J1. He was also selected "Asian Young Footballer of the Year" award in 2003. In 2005, he moved to Spanish La Liga club Mallorca on loan. Although the club results were bad, Mallorca finished at 17th place and remained La Liga in 2004/05 season. Although he played many matches as substitute forward in 2005/06 season, his opportunity to play decreased from April 2006. In June 2006, he returned to Cerezo Osaka. At then Cerezo won only 1 match in 13 matches and at the bottom place of 18 clubs in 2006 season. Although he played as forward all matches excluding suspension, Cerezo finished at the 17th place and was relegated to J2 again. In 2007, he moved to J1 club Vissel Kobe. He played many matches as left midfielder in 2 seasons under manager Hiroshi Matsuda and scored more than 10 goals both seasons. In January 2009, he moved to Wolfsburg of the Bundesliga. Wolfsburg won the champions 2008/09 season which was first major title in the club history. In June 2009, he returned to Japan and re-joined Vissel Kobe. He played as forward all 19 matches and scored 8 goals in 2009 season. Although his opportunity to play decreased for injuries in 2010, he played as regular player until 2012. However Vissel finished at the 16th place in 2012 season and was relegated to J2 League. In 2013 season, he moved to Kawasaki Frontale. He played more than 30 matches every season and he became a top scorer for 3 years in a row until 2015 season. In 2017, he moved to FC Tokyo. In 2018, he returend to Kawasaki. However he could not as regular player and he mainly played as substitute forward. In June 2018, Okubo moved to Júbilo Iwata. In 2020, he moved to J2 League club Tokyo Verdy. In 2021, he moved to his first club Cerezo Osaka.