The 23-year-old central defender has been training with the Black Cats, and he has already played in a fixture behind closed doors.
Sunzu was forced out of the action with a slight knee injury, while he will also head out on international duty for a World Cup qualifier in Lesotho.
Premier League rivals Reading pulled out of the race for Sunzu in January due to a contract dispute with TP Mazembe.
The Congolese club eventually resolved that Sunzu was under contract until 2015, rather than being a free agent, but Mazembe have given the player permission to train with Sunderland.
O'Neill told the Sunderland Echo: "He played a little game the other day, but he had a swollen knee before he came here and he had to come off in the game.
"That will stop him training for a week at most. And then he'll be going away to play during the international break so we'll monitor him during that game.
"But I haven't seen enough of him yet to make a decision on him."