The Scotland international joined the Black Cats last year on a short-term deal, but that came to an end in January.
O'Neill was keen to keep him, but he did not stand in his way when the then Nottingham Forest manager Alex McLeish showed an interest and was ready to offer the 29-year-old a long-term deal.
However, McLeish has now left the City Ground - so a move back to Sunderland for McFadden could be back on the cards.
O'Neill told the Sunerland Echo: "We let James' situation run past the transfer deadline and I think he went down to Nottingham Forest before McLeish departed.
"But obviously events have changed that situation and I don't know what's happened since then. I'll speak to the agent though and see where we stand."
As McFadden is a free agent, he can re-join Sunderland outside the window.