After a 6-0 win at Parkgate last Saturday, Steve Evans men had another very useful workout on their pre-season visit to the Algarve before they return home on Thursday.
Jaaon Taylor gave the Millers the lead on 10 minutes as Oldham were placed onto the back foot early on.
Winger Gareth Evans missed the chance to double the lead as his spot kick hit the bar just five minutes later.
The Latics finished the first half the stronger side but were unable to force an equaliser before the break.
Second half strikes from Alex Revell (55) and substitute Daniel Nardiello (71) put the Millers into a comfortable 3-0 lead before Oldham's M'Changama pulled a goal back with 15 minutes to play.
Team details to follow.