Jacques Maghoma's low shot on-the-turn 20 minutes from time brought the visitors a first away point in 2013 after a barren run without a goal on their travels this year.
James Constable had earlier headed Oxford in front from Tony Capaldi's corner on his first game back following a four-match suspension.
But conditions at the Kassam Stadium, where rugby side London Welsh also play, were atrocious, with the pitch totally churned up along one side.
"I thought we deserved to win in what was a tight game, especially in the first half," he said.
"The majority of our games from now until the end of the season are going to be at home, so we've got to find ways to win at home.
"We've simply got to find a way to win games at home, because conditions aren't going to change.
"How we do at home is going to determine the rest of our season."
Oxford lost veteran defender Michael Duberry in the first half and Wilder added: "Duberry strained his hamstring, and that was a blow to us."