Michael Doyle and Dave Kitson were on target for the Blades, who have now kept five consecutive clean sheets.
United had the better of the first-half chances, with Harry Maguire heading off target and also forcing a reaction save from Oldham keeper Alex Cisak.
Jose Baxter was a threat for Oldham, but his two efforts from the edge of the box flew narrowly over.
United went in front after 57 minutes when Dominic Poleon's shot was half-saved by Cisak and the rebound was converted by Doyle from 12 yards.
Oldham almost levelled but Matt Smith headed Lee Croft's cross inches wide before seeing his 18-yard shot palmed out by George Long.
United sealed the points after 86 minutes when Cisak charged off his line and failed to gather Tony McMahon's free-kick, giving Kitson the chance to head into an empty net from 14 yards.