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."
Oxford United 01 Clarke 03 Newey 06 Wright 13 Hunt 19 Mullins 14 Hall (Williams - 52' ) 16 Whing 17 Rose 18 Davies (Kitson - 46' Booked ) 09 Constable 11 Potter Substitutes 21 Crocombe 04 Raynes 15 Williams 20 Marsh 35 O'Dowda 08 Kitson 37 Ruffles Wycombe Wanderers 01 Ingram 02 McCoy 03 Dunne Dismissed 06 Johnson 30 Doherty Booked 20 Craig (Pittman - 69' Booked ) 23 Arnold 28 Knott Booked (Spring - 89' ) 04 Scowen 07 Morgan Booked (Kuffour - 76' ) 11 Wood Substitutes 41 Harrison 05 Stewart 09 Spring 22 Kretzschmar 16 Pittman 18 Kuffour 29 McClure