Oxford boss Chris Wilder dubbed his side's performance in the 2-0 loss to Scunthorpe as their worst of the season.
The Iron were comfortable victors here as a goal in each half from Deon Burton and David Syers sealed victory.
Burton also missed a penalty, saved by Ryan Clarke - but he was not alone - as Oxford's Deane Smalley similarly saw his late spot kick stopped superbly by Sam Slocombe.
it was a second successive home defeat over the festive period for Oxford and they slipped out of the automatic promotion places and down to fifth.
"We didn't deserve anything from the game. We were second best all afternoon," said Wilder.
"It was by far and away our worst performance of the season.
"We came across an excellent team in great form.
"We gave away a poor goal to start with, and the second one was a mistake from us as well.
"We made poor decisions in every area of the game, in every area of the pitch, whereas they made good decisions everywhere, and they were a threat at the top of the pitch, and fully deserved their win.
"My players know a performance like this is unacceptable to me and unacceptable to the supporters; I don't want to see that again.
"I think we've undersold everybody. I don't think we've given it an unbelievable go, like we do in every match, and that is a big disappointment."