Gerrard, 30, returns after being released by the Latics in June 2016 and has signed a two-and-a-half year deal.
Amadi-Holloway, 23, was on loan at Oldham last season but has now moved on a permanent deal from Fleetwood.
Former Liverpool trainee Ngoo, 24, has agreed a deal until the end of the season after a trial spell.
All three players are able to play in Saturday's game against Shrewsbury.
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Oldham Athletic have re-signed defender Anthony Gerrard and striker Aaron Amadi-Holloway and brought in striker Michael Ngoo on free transfers.