Oxford scored three goals in eight minutes at the start of the second half to crush struggling Gillingham 3-0.
Jack Payne's audacious flick with his heel to turn in Rob Hall's cross in the 48th minute began the rout.
Payne then turned provider six minutes later with a pass for Joe Rothwell to race through and slot past goalkeeper Tomas Holy from the edge of the area.
Gillingham, still looking for their first league win, had not recovered from that blow when Hall ran on to Jonathan Obika's backheel and drilled an angled shot under the keeper.
Oxford might have added others as Wes Thomas and Gino van Kessel squandered great chances.
The first half was much tighter and Gills should have gone in front in the sixth minute, but the unmarked Conor Wilkinson headed wide from seven yards.
The U's were then guilty of a similarly glaring miss when, after great work from Hall, Obika blazed over.
Gills' first effort on target was Lee Martin's shot after 73 minutes that Simon Eastwood saved.