The Kenya international was surprisingly released by the Team of Choice prior to the start of the current campaign after years of serving the club.
Onyango was one of Maritzburg's key players as the team narrowly avoided relegation to the National First Division (NFD) last season.
The 25-year-old player, who is now a free agent, was previously linked with Mamelodi Sundowns but that was when he was still contracted to Maritzburg United.
Nevertheless, the bulky player is said to have begun training with the Buccaneers this week as he looks to prolong his stay in the PSL.
Pirates were expected to sign a central defender before the closing of recent transfer window having been linked with Bidvest Wits captain Thulani Hlatshwayo.
However, the Soweto giants' interest in the no-nonsense defender cooled off after he signed a new long-term contract with the Students.
Onyango, who has been playing in South Africa since 2012, will be hoping to impress Pirates' caretaker coach Rhulani Mokwena and earn himself a deal at the club.
When he first arrived in 2012, Onyango played for former PSL champions Santos from Kenyan side Tusker before Maritzburg United took him away in 2015.
If he joins Pirates, Onyango will reunite with his former coach at Maritzburg United, Fadlu Davids, who is now working hand-in-hand with Mokwena.