After losing at Yeovil last weekend, Ole Gunnar Solskjaer and Warren Joyce’s men turned in a much-improved performance against an Oxford side pepped-up by an impending return to Football League action.
Despite impressing, however, the newly-promoted League Two team could have been behind inside five minutes. James Chester was felled by Mark Creighton inside the home penalty area, but Magnus Eikrem’s spot-kick was saved by goalkeeper Ryan Clarke.
United's Norwegian playmaker atoned in the 44th minute, though, by drilling home a 20-yard free-kick. Incredibly, the hosts found time to restore parity before the interval, as skipper James Constable struck a powerful low shot past Sam Johnstone.
The Reds were penned back for long periods after the break and Matt Green and Simon Heslop both came close for the hosts, but United snatched victory with virtually the last kick of the game.
Oliver Norwood displayed his set-piece prowess with a magnificent in-swinging cross from the left, and Ajose was on hand to thump home a volley for his third goal in as many pre-season starts.
United: Johnstone; Wootton, Fryers (Dudgeon 46), Chester, Gill; Eikrem (Lingard 78), Possebon, Morrison (Tunnicliffe 83), Brady (Stewart 46); King (Norwood 68), Ajose. Subs not used: Devlin, Cathcart.