Looks like it takes ~3 hours and a couple of changes. http://ojp.nationalrail.co.uk/service/timesandfares/BNY/Oldham/140315/1400/arr/140315/1730/dep
Train to Manchester piccadily then a 10 minute walk up to market street tram stop then straight to oldham isn't it?
National rail site seems a bit misleading to me if your going from Manchester it has you going from Piccadilly to Victoria which takes longer than just nipping up to market street stop and jumping on straight to oldham in 30 minutes or so...