The visitors totally outclassed their hosts in a one-sided game that was effectively over as a contest well before half-time.
Oldham started much the better side and their enterprising play left City chasing shadows.
Teenage wideman Chris Taylor tormented Bradford's makeshift left-back Mark Bower and the 19-year-old put across a series of testing crosses.
It seemed only a matter of time before Oldham would take the lead and they did on 15 minutes largely thanks to a blunder by City stopper Donovan Ricketts.
The impressive Richie Wellens was allowed time to fire a shot goalwards and Ricketts seemed to have it covered but split the shot and Paul Warne was on hand to tap in.
City barely had time to recover before they found themselves two down. Again Wellens was the architect sliding through a superb ball to striker Luke Beckett who finished neatly.
Marc Bridge-Wilkinson came close with a free-kick for the Bantams but that was as near to Oldham's goal as City got before the boos began to ring around Valley Parade.
But things got even worse for the home fans as Richard Butcher made it three on the half-hour mark.
Again Bradford's defending was shambolic as they allowed Warne to get to the byline. He cut the ball back to Butcher who sided-footed in.
Banatams boss Colin Todd made two changes just before half-time, hauling off Owen Morrison and Steve Claridge in attempt to breathe some life into his beleaguered side.
Bradford did briefly threaten an unlikely comeback when on-loan striker Aaron Wilbraham headed home Ben Muirhead's cross.
That was as good as it got for Bradford who, despite enjoying more possession, barely threatened Lee Grant in the Oldham goal.
And it was the visitors who put the gloss on their victory when Beckett coolly rounded Ricketts to make it four eight minutes from time.