Senegal international Mangane is a towering central defender and O'Neill believes he will be a crucial addition to the squad, following his arrival on loan from Al-Hilal.
However, the Black Cats boss admits it may take some time before the 29-year-old is completely ready to be thrown into Premier League action.
"He's got a great physique and presence and certainly has what it takes to do the job," O'Neill said in the Daily Star.
"But it's fair to say that Kader's a little bit behind the rest of the lads in terms of fitness.
"Once he's fully fit, he'll be pushing for a start in a position where we've been really stretched of late."
O'Neill added: "Whenever a loan signing comes in from abroad, part of the deal is that there has to be an option at the end of the deal for a permanent signing and that's the case with Kader."