The much maligned Valley pitch was far from perfect, looked badly worn in places, but was definitely in a playable condition.
Chris Powell, on his third anniversary as Charlton manager,, fielded a stronger than usual line-up for a cup match. Ben Hamer got his first start since the Queens Park Rangers game in late November.
Chris Solly and Dale Stephens were not risked, so Lawrie Wilson dropped back into defence and Johnnie Jackson partnered Jordan Cousins in the middle. Bradley Pritchard and Jordan Cook were on the flanks with Yann Kermorgat and Simon Church up front.
The line-up in the Directors’ Box was almost as interesting. Katrien Meire was flanked by Keith Peacock and Nigel Adkins, with Martin Allen and Michael Slater just behind and Martin Prothero and Ben Kensell behind them.
Oxford adapted to the conditions better than the Addicks and stunned their hosts with two first half goals.
First, a deflected effort from Johnny Mullins wrong-footed Hamer after 13 minutes. Then, eleven minutes later, Jordan Cousins lost possession in the middle of the park. The ball was played to Scott Davies on the right and the midfielder rifled the ball into the right-hand corner of Hamer’s net.
Charlton gathered momentum in the second half and eight minutes after the interval, Michael Morrison halved the arrears. He met Cook’s left-wing corner with a header into the roof of the net.
Danny Green, then Callum Harriott and Joe Pigott entered the fray as Powell chased a second goal and it was Green who created the equaliser. His cross fron the right was put away by Kermorgant, in acrobatic fashion.
There was still time for Harriott to have a header cleared off the line, but the teams must do it all over again, at the Kassam next week.
Charlton: Hamer; Wilson, Morrison; Wood, Wiggins; Pritchard (Green 67), Cousins, Jackson, Cook (Harriott 78); Church (Pigott 78), Kermorgant.
Subs (not used): Pope, Hughes, Evina, Dervite.
Goals: Morrison (53), Kermorgant (82).
Oxford: Clarke; Hunt, Raynes, Wright (c), Newey;Rigg, Davies (Williams 87), Mullins, Marsh (Smalley 53); Ruffels, Constable (Kitson 78).
Subs (not used): Rose, Long, Bevans, O’Dowda.
Goals: Mullins (13), Davies (24)m
Referee: Steve Martin (Staffordshire).
Attn: 5,566 (870 Oxford).