The 20-year-old stopper, who's made two first-team appearances for United this term, will gain valuable experience at Boundary Park, although he faces stiff competition from current custodian Dean Brill.
The Latics, 13th in League One, host Swindon Town this weekend and Oldham boss Paul Dickov says he's looking forward to Amos' arrival.
"I'm absolutely delighted," he told the Manchester Evening News. "Ben's a top young English keeper and I'd like to thank Sir Alex Ferguson and [Reds goalkeerp coach] Eric Steele for helping it to happen.
"Dean Brill has done well for us but we feel Ben's arrival gives us extra options."
Amos, who began the season as the Reds' third-choice goalkeeper, slipped down the pecking order after Denmark international Anders Lindegaard, 26, signed for United this month. The loan spell at Oldham should, however, provide Amos with first-team opportunities and ensure his development continues.