Harewood moved to Villa from West Ham in a £3million deal during the summer but has so far only started the Carling Cup matches against Wrexham and Leicester.
O'Neill said: "Marlon knew it was going to be difficult. We already had players whose ability I knew about. Carew, Agbonlahor, Moore, for a start, never mind Shaun Maloney who might also play up front.
"Marlon knew what he was coming into here and he's still not been able to play two or three successive games because the lads are doing fine and well enough. That is what he is crying out for, a run in the side, and it is difficult for him being in a stop-start mode at present."
The Villa boss added; "If Marlon wants to give it a bit of time, I am hoping he will settle down and into proceedings here. But like everyone, he wants to get up and running and so far his starting games have been confined to the Carling Cup."