Kemar Roofe smashed Michael Appleton’s men into a first-half lead before County stalwart Michael Flynn saw red.
Dack could only guide the Exiles to a ninth-place finish and hands over managerial duties to former England captain Terry Butcher.
The three-points meant Oxford ended the season in 13th position.
Roofe fired the Us in-front in the 31st minute. Former Newport man Danny Rose slipped a through ball to the striker who found a pocket of space, and he rifled a right-footed shot into the top corner from 12 yards out.
And things got worse for the home side in first-half added time when Flynn was shown a red card on his 100th appearance for the club.
The centurion left a crunching, studs-up tackle on Michael Collins before getting to his feet and head butting the Oxford midfielder.
Aaron O’Connor had a late chance to level but his close-range shot was blocked.