If you're having a drink this route would work well from Oxford. Lots of pubs in Ealing Broadway and down St Mary's Road which goes in the right direction for the ground and is on the 65 bus route (which goes from Ealing Broadway and passes Griffin Park). You could easily drop into a number of pubs down the road towards Brentford then get on the 65 anywhere along it.
From Oxford this route would probably work best anyway, you can do Oxford to Ealing Broadway in an hour (my partner's from Oxford so I've done this route a couple of times to visit her parents) with that change at Slough you mentioned and the 65 starts its route directly outside Ealing Broadway Station just to the right. Far better option to go to a few pubs first though obviously, doing football without pubs is never right.