James Constable crowned a man of the match display with the 16th minute winner to take his tally for the term to 11 goals.
Constable met right-back Damien Batt's pacy angled drive into the six yard box for the game's only goal.
Argyle keeper Jake Cole made a superb one-on-one stop to deny Oxford top scorer Tom Craddock following a poor backpass from Argyle right-back Durrell Berry in the 53rd minute and seconds later Constable hit the upright.
The closest Argyle came to a leveller was in the 63rd minute when forward Reuben Reid's sidefoot volley was headed off the line by Oxford skipper and central defender Jake Wright.
"You wouldn't think we were a team unbeaten in four," he said. "After a good win last week we come into a home game against a team who haven't won in five you would think we would be the confident ones going out on the pitch.
"I am very disappointed, especially with the first half performance. We just look frightened and I don't know why. There wasn't any energy and it was a great opportunity to push away from one or two teams down near the bottom.
"We just did not get going at all. That should never be the case. I keep saying we look bright in training and then we go out there on a Saturday and perform like that, it's probably the sloppiest we have been.
"We have been sloppy all over the place and I am not happy with that. It's like the goal. It's just a basic throw-in and they end up scoring a goal from it and then some of the back passes when we were chasing the game, and some of the decisions we made were criminal.
"There are one or two players who want to come into in the team and so there might be one or two changes."