O’Neill has already snapped up Alfred N’Diaye from Bursaspor in a £3m deal this month, with Kader Mangane also moving to Sunderland on loan until the end of the season.
Despite the two new arrivals, the 60-year-old wants to strengthen his squad further after Sunderland were beaten 2-0 by Bolton Wanderers in their FA Cup third-round replay.
“David Meyler has left to join Hull because he wanted to play first-team football and Ji [Dong-won] has also gone on loan to Augsburg until the end of the season,” O’Neill told the Journal.
“There was room in the squad anyway, but now there’s even more space to be filled.
“We’re still just two out and two in. I would still be looking to get some more people in before the end of the month. And I would certainly want more people in before we would even think about letting anyone else out.”