The duo have left for Pirates as part of a swap deal which sees Justice Chabalala, Donald Mokondelela, Sello Japhta and Nkosinathi Mthiyane move in the opposite direction to the Chilli Boys.
Okwuosa and Van Heerden are expected to start training with Bucs on Monday.
"We are happy we have replaced those players with equally quality players," Mpengesi tells KickOff.com.
The Sea Robbers previously looked to recruit Okwuosa in the off-season, but changed their mind at the last minute.
Former Bafana Bafana fullback Van Heerden spent the past six months with United afer being snapped up from NFD outfit AmaZulu.
Chabalala has made eight league starts for the Buccaneers this campaign, while Mokondelela, Japhta and Mthiyane have been peripheral figures at Mayfair.