Warren Joyce’s side now lead the group after winning all five games, with City in second, having won all four of their matches.
Victory for United in next Thursday’s mini-derby against the Blues at Altrincham’s Moss Lane will clinch their place in the final.
United were rarely troubled at Carrington and scored all their goals in a 10-minute spell just before half-time.
Jesse Lingard pounced to score from close range in the 35th minute and then provided the pass for Will Keane to increase the Reds’ lead.
Skipper Davide Petrucci made it 3-0 shortly before the break after Oldham keeper Dean Bouzanis had blocked Reece Brown’s cross.
The visitors went close to grabbing a consolation in the closing stages but Matt Smith’s header went just wide.
UNITED: Kuszczak, Vermijl, Giverin, Fornasier, M Keane, Thorpe, Fabio, Brown (McGinty 75), W Keane (Massacci 55), Petrucci, Lingard (Leao 55). Not used: Johnstone.
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