The Latics moved back up to third place while the Bulls are bottom of the division and still waiting for their first away point of the season.
Hereford fell apart in the opening 45 minutes and manager Graham Turner slammed their performance, dubbing it "unacceptable".
Oldham made the ideal start when left back Daniel Jones burst into the visitors' box and hit a powerful shot which keeper Darren Randolph could not hold.
The ball fell to striker Lee Hughes, who drilled low into the net from only four yards to register his sixth goal of the season.
Hereford had a rare attack which saw Toumani Diagouraga denied by a block but they went two nil down on the quarter hour after veteran defender Bruno N'Gotty was deemed to have tripped Hughes inside the area.
Referee Rob Shoebridbe pointed to the spot and Andy Liddell made no mistake as he sent Randolph the wrong way.
Oldham effectively killed off the game on 36 minutes when the impressive Jones, on loan from Championship club Wolves, raced onto a fine pass by Chris Taylor.
Jones cut into the box and finished off the move in spectacular style with a blistering 16 yard shot inside the far post.
Hereford improved after the break and Steve Guinan tested substitute keeper Greg Fleming but Oldham were always in control.
They missed several more chances before Hereford went down to ten men when Stephen O'Leary, who had just been booked, was shown a red card for his high tackle on Mark Allott.
Oldham rubbed salt into the wounds as midfielder Danny Whitaker found space 14 yards out to slide his shot into the bottom corner and seal Hereford's misery.