But the opening 20 minutes were fairly even, and it was Oxford who seemed to be first to the ball and certainly not afraid to assert their physical presence.
However, there was a scare for the Oxford fans in the fifth minute after Jason Price's cross was deflected in the area and Paul Wanless put it inches wide of the near post.
Five minutes later Richie Foran was lucky to stay on the field after a very late challenge on Stuart Green, getting away with a yellow card.
After 25 minutes Oxford enjoyed a spell of pressure in City's box but could not get a shot on target.
After 28 minutes, Stuart Elliott brought a save from Andy Woodman after being played in by a great ball down the middle from Jason Price.
A minute later Ben Burgess's header from an acute angle was saved at the second attempt by Woodman.
Allsopp and Elliott both headed wide, as did James Hunt at he other end in the minutes before half time.
But the second half saw Oxford's resolute defence in tatters after a great run by Elliott down the left played Burgess in, who from just inside the box placed an excellent looping shot which landed in the bottom left corner of the net, completely beating Woodman.
Just five minutes later Allsopp made it two after a fortuitous deflection in the box and a minute later bagged his second from the left side of the box, cooly shooting underneath Woodman, who should have done much better.
On 77 minutes, Steve Basham's shot was hacked off the line but there was no stopping his rebound. Then two minutes later City widened the gap again when Crosby nodded into his own net from a crowded far post, with Burgess initially claiming the glory.
And in injury time Matthew Bound's powerful free-kick beat Boaz Myhill to make the score a little more respectable for the visitors.