New Oxford manager Liam Manning saw his relegation-threatened team beaten 3-2 by a Louie Sibley-inspired Derby at the Kassam Stadium.
Manning, who will be officially unveiled on Monday, watched from the stands as Oxford took the lead before League One play-off chasing Derby hit back impressively.
The U's opened the scoring with Sam Long heading in James Henry's free-kick at the back post after 14 minutes.
Derby took just nine minutes to equalise with Korey Smith firing in a low cross from the left that Sibley converted high into the net.
Sibley completed his brace five minutes before the break with a right-footed shot from 15 yards finding the top-right corner.
County sealed the points with a third just after the hour mark when James Collins headed in Jake Rooney's right-wing cross, the ball going in off defender Long's back.
Substitute Kyle Joseph pulled one back with a precise left-footed shot at the death but it was not enough and Oxford's winless run now stands at 10 games.