It is a great city. Yeah, I did mean the "Coven" - perhaps my eye sight wasn't too good whenever I went there on a psytrance night!
I'm sorry to hear that it has closed, it was really the best place for that sort of music, but it's been almost 5 years since I was there. Other than that, the Oxford Tube is really good for going to, and coming from London - and there are certainly plenty of Hardcore (legal and illegal) parties to be found in London!
I'm not really in to hardcore - more in to bands and the occasional psytrance night - so maybe I'm not the best person to ask, but I don't think that Hardcore is that popular in Oxford. Having said that, there is a nightclub at Oxford Brooks university which is quite good, and they sometimes have decent stuff on. You don't have to be a student there to go (but you may have to be "a" student, on some nights, I think).
They do have some really good bands at that club I'm thinking of on Cowley, if you're into that kind of stuff. There are 3 or 4 clubs in town, but I think you'd be lucky to find Hardcore at any of them, to be honest - maybe Park End (that's what it was called when I was there, anyway - maybe things have changed names now).
I'd look into taking the Oxford Tube to London, if I were you. If the Coven is closed, then I can't really think that there would be much happening in Oxford itself.