I suppose it depends on what you are trying to solve.
If it's "a link from Manchester to Oldham" then what would you be offering that a six minute tram frequency doesn't? A significantly faster journey time? How frequent would it have to be for people to swap a six minute tram frequency and "half hour" journey time? (I say "half hour", since the tram has a lot more stations in both Oldham and central Manchester than heavy rail could provide, so you can count "King Street to Victoria" if you want to make the tram more like twenty five minutes or you can count "Mumps to Deansgate" if you want to make the tram more like forty minutes - with the proviso that heavy rail wouldn't penetrate either centre as well)
The great thing about Oldham (like Sheffield) is it's proximity to the wilderness - but the fact that the population density is so low as you go slightly beyond it counts against it in public transport terms. So would it be just a Manchester - Oldham route (and no further)?
I've suggested before that, if we are building HS3/ NPR to a brand new alignment, with lots of tunnels, then there's no reason to follow the same Victorian routes that current trans-pennine trains use - we could go in a pretty straight line from Manchester to Leeds. And, instead of spending billions on a slower route so that you can tick off Bradford too, you could follow a pretty straight line (e.g.
https://www.freemaptools.com/how-far-is-it-between-manchester_-england-and-leeds_-england.htm) with stops at Oldham, Brighouse and maybe Gomersal Park & Ride - that would provide a much faster route that the proposed Bradford line but allow the new railway to serve new places and provide new links (rathe than just shaving some time off existing direct journeys)
So, for me, serving Oldham involves either "mega-expensive high speed trans-pennine route" or "do nothing, stick with the high frequency tram service" - there doesn't seem any middle ground, since you'd be spending a lot of money for only marginal benefit.
