Johnny Mullins' injury-time equaliser helped maintain Oxford's unbeaten start to League Two against 10-man Wycombe.
The visitors took the lead when Dean Morgan nodded home and went close through Neil Arnold's 40-yard lob.
Wycombe's Charles Dunne was dismissed for an elbow and Oxford levelled when Danny Rose's free-kick was misjudged by Matt Ingram and bounced straight in.
Morgan restored Wycombe's lead from the spot after he was fouled by David Hunt before Mullins drilled home late on.
VIEW FROM THE DRESSING ROOM
Wycombe manager Gareth Ainsworth told BBC Three Counties Radio:
"It was a great performance by the boys. We didn't turn up at all last week and this week I made sure the lads knew what they were playing for.
"For the want of a couple of decisions which I disagreed with, we might have held on and taken the [three] points, but I'm not going to gripe about any refereeing because it's a part of the game.
"Billy Knott had an immense debut. He has energy, passion, drive, everything that I like in a player, and on top of that he's got Premiership quality with his left foot."
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