Whilst I am skeptical about the prospects for many of the UK's marginal airports, I believe Oxford could carve out a niche for itself due to its location. Although Heathrow is only 40 miles away, it is a congested 40 miles by road and there are no direct trains. Luton is about the same distance, but the roads are very poor. Birmingham is accessible via the M40 but is 66 miles away and has expensive parking. Southampton is no closer, nor is Bristol.
Oxford would be the closest airport for quite a large population, and in a very affluent part of the country. There are lots of high-tech companies in the local area, especially in the medical and pharmaceutical sectors.
If parking remained reasonably priced, and the airport stayed uncongested, I could see connecting flights to a European hub working, as well as perhaps Edinburgh.