Millwall slipped to a disappointing 3-0 defeat at home to Oxford but remain in the Sky Bet League One play-off places.An early Conor McAleny double and Josh Ruffels' second-half header did the damage and lifted the U's to ninth.Despite early pressure from the hosts, the U's took the lead after five minutes when McAleny fired in from the edge of the box.The winger, on loan from Everton, then doubled the lead, cutting in from the left and curling into the far corner.Neither side could muster another shot on target during a frustrating first half for the hosts.Steve Morison was put clean through on the hour mark but fired straight at Simon Eastwood, and Oxford sealed victory when Ruffels nodded in Chris Maguire's 70th-minute corner at the far post.Millwall are one point above seventh-placed Southend.