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 early on 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.
A Second half volley from Alex Revell (55) from David Nobles cross and a goal from fellow substitute Daniel Nardiello (71) after being set up by Kari Arnason put the Millers into a comfortable 3-0 lead, before Oldham's M'Changama pulled a goal back with 15 minutes to play.
Millers: Shearer, Hunt, Mullins, Sharps, Wilson, Ainsworth, Arnason, Taylor, Evans, Revell, Odejayi
Bench: Warrington, Bradley, Noble, Nardiello, Pringle, Rose, Denton.