From my experience, if you're a fairly good actor, and you make the commitment to get involved, you can do so at a university level relatively easily - as you say, OUDS centralises everything, and they send out weekly newsletters letting you know of different productions that need actors, as well as venue and funding opportunities (if you're into the producing side). The first opportunity to get involved will be drama Cuppers, which is where colleges each perform a play in the BT Studio (a black box theatre), and you get ranked by some judging panel, and the best plays and actors are given awards. Anyway, point is you don't need to go to a college with a theatre to get involved with drama, and I'd say it's probably better to get involved across the university just from the perspective that you're not always acting with the same crowd.
As for the specifics of your college requirements, I was at Merton, and it does have well located and nice, cheap accommodation for all three years, and you'll likely have en-suite in your third year (I had an ensuite for 2 out of the 3 years I chose to live in college, as I had a nice committee room in my second year). Also, it's worth noting that the ballot is entirely 'fair' in the sense that it doesn't give preference to scholars, and given that acting (I think and have experienced, at least!) is probably the most destructive extra-curricular timewise, it might actually be an important consideration! Also, I don't even know if it's open for theatrical use, but Merton has recently finished building a lecture theatre, so there's (possibly!) the possibility there if you did want a college with that sort of space. The Chapel and Gardens have mostly been used for productions that I've been involved in, and they've sufficed!