The deal was for an initial one-month stay and it ends after Sheff Wed's match at Leicester tomorrow.
Sunderland only have two top Premiership strikers, and an injury to either would put O'Neill in a difficult position.
Martin O’Neill: “I’ll have a look at it and see where we are.
“The idea was for him to get some competitive games in, to help himself and help Sheffield Wednesday along the way.
"At least he’s played some games.”