The Nigerian defender had been excellent all night long, bottling up Mame Niang who was cautioned and substituted late in the game.
But then, rather cruelly, Okwuosa turned the ball into his own net following a Thabo Mosadi cross right at the end of optional time.
Chippa coach Farouk Abrahams was understandably livid after the game, having seen his team waste numerous scoring chances only to end up losing.
The Chilli Boys' first attempt on goal arrived after 12 minutes, when Washington Arubi had to parry away a stinging Diego Brown free-kick.
Then, on the half-hour mark, Ayanda Lubelo skied a shot way over the crossbar for the home side.
In the 37th minute, Chippa missed another really good chance when Welcome Qalanto got into space, but he rushed his shot and volleyed wide of the post.
Chippa's best chance of the first half came when Lubelo turned his marker and hammered a left-footed shot at goal, but Kassaly Douada saved well.
Okwuosa then made a telling slide tackle on Niang in the 57th minute before Chippa substitute Mkhanyiseli Siwahla put a volley wide of goal after 72 minutes, and it seemed like the visitors would score.
But then in the final minute Okwuosa could only direct Mosadi's cross into his own net, for a cruel end to the match for the visitors.