The Black Cats could see their defensive options limited at the end of the season with two players out of contract and two loan signings due to their return to their parent clubs.
Centre-backs Titus Bramble and Matt Kilgallon see their contracts expire in the summer, while loan signings Danny Rose and Kader Mangane are set to return to Tottenham and Saudi Arabian side Al-Hilal respectively.
O'Neill knows he must bolster his defensive options and he concedes the full-back spots are his biggest two areas of concern with Phil Bardsley the only recognised full-back on the books at the Stadium of Light.
"Phil is the only recognised full-back we have at the football club and it's something we have to look at in the summer," O'Neill told the Sunday Sun.
"Craig Gardner has dropped back from midfield to play there while Phil has been out - and has done very well - but he's not a specialist.
"Ideally, we need competition for places but we also need players playing in their proper positions, and the positions that they feel most comfortable in."
O'Neill added: "Since I've been here at the football club, we haven't had a specialist left-back.
"Kieran Richardson played there last season but he wouldn't really consider himself a left-back.
"Danny Rose has been here on loan and he has done really well when he has played, but still that is a position that we would have to look at in the summer."