Oiwa was born in Shizuoka on June 23, 1972. After graduating from University of Tsukuba, he joined J1 League club Nagoya Grampus Eight in 1995. From first season, he played as center-back with Alexandre Torres for long years. The club won the champions 1995 and 1999 Emperor's Cup. In Asia, the club won the 2nd place 1996–97 Asian Cup Winners' Cup. He moved to Júbilo Iwata in September 2000. The club won the champions in 2002 J1 League and the 2nd place 2000–01 Asian Club Championship. In 2003, he moved to rival club Kashima Antlers, biggest rival competed for the champions with Júbilo. He played as center-back with Yutaka Akita or Daiki Iwamasa. Although his opportunity to play decreased from 2007, the club won the champions for 3 years in a row (2007-2009). The club also won the champions 2007 and 2010 Emperor's Cup. He retired end of 2010 season.