Opiyo began his youth career with
Tusker and signed for
Thika United in January 2008. After only one year with Thika United, he moved to
Gor Mahia in February 2009 on loan, and his loan spell with Gor Mahia was set to continue until 2010. In 2011, he returned to Thika United for the remainder of the year, before rejoining
Tusker for the 2012 season helping the side to clinch their ninth league title. He moved to
A.F.C. Leopards in 2013, helping the side to a second-place finish in the league and a ninth
domestic cup. He scored the lone and winning goal against arch-rivals
Gor Mahia. On 11 March 2014, Opiyo completed a move to Finnish top-tier side
FF Jaro in a two-year deal. He made his debut for the club in their fourth-round
2014 Finnish Cup match against
FC Hämeenlinna on 15 March, but could not help prevent the club from a 2–1 loss and an early exit from the tournament. After flirting with AFC Leopards in the June transfer window, Kenyan international midfielder Opiyo sealed a move to Qatar second division side
Al-Markhiya SC on 25 July 2016, a team he led to promotion at the end of the season. On 26 July 2018, Opiyo signed for
SJK. He left the club again at the end of 2018. In August 2019, Peter Opiyo and Nigerian
Uche Kalu signed a contract with
FC Altyn Asyr from
Turkmenistan. They were the first foreign footballers in the history of the club and the first legionnaire in the
2019 Ýokary Liga. Despite having a contract, paperwork complications saw him fail to join the team and in mid-November 2019 he issued a termination notice and consequently ended his engagement with the club in early December to become a free agent. After weeks of training with promotion-chasing
Nairobi City Stars, Opiyo joined the second-tier side in January 2020 on a one-and-a-half-year deal. He went on to renew his stay with the club with separate one-year deals to feature in the 2021–22, and 2022-23 FKF Premier League seasons. ==International career==