Black Cats forwards Stephane Sessegnon and Steven Fletcher have only recently formed a partnership, after the latter's arrival from Wolves, and while O'Neill acknowledges that, he knows they need to put right a lack of fitness up front.
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"We're so many weeks into the season that even I'd expect to be match-fit by now.
"But there's no getting away from the fact that it's there and it's something we are working on.
"Steven Fletcher hasn't had a pre-season, but he's much fitter than he was against Swansea a couple of weeks ago and he almost finished the game against Liverpool."
But despite Fletcher improving, O'Neill admits that Sessegnon isn't playing at his best at the moment, and the Sunderland boss is looking forward to watching new loan signing Danny Rose improve over the coming weeks as his fitness gets better.
O'Neill added: "Stephane Sessegnon? It's just not happening for him at the moment, then there was Danny Rose, who I think will be a decent player. When he gets his fitness, he will impress."