"We are looking. There are a couple of positions we need to top up in," Wilder told BBC Oxford.
"We need competition at left-back. In the middle we lost [Dannie] Bulman at Barrow and we were light there."
He added: "We lost Damian Batt and Adam Chapman had to cover right-back and we maybe need a bit of strength in there."
"We haven't got a massive squad, we are not carrying 25 or 26 players. We are carrying a quality tight group of 19 or 20."