Chris Wilder rued a key decision going against his Oxford side after Chesterfield snatched a last-gasp 2-1 win.
United scored in the first minute when John Mullins headed in a corner but Liam Cooper replied in first-half stoppage time before Chris Atkinson won it at the death.
"I don't know how for the life of me he (referee Dean Whitestone) found five minutes of extra time in the first half and there were a couple of penalties for us as well," he said.
"We felt comfortable and it could have gone either way, but we've come away from the game we've had big periods in and finished pointless.
"Yes, we should have defended the first goal better and should have finished the chances off in our dominant first half-hour because it was all us but I do feel the officials have played a big part is us not getting something out of the game.
"I think we've worked extremely hard and at the very least I think a point was deserved."