The Vue cinema at the Kassam Stadium is showing many Olympic events, as well as the opening ceremony. You must register for free tickets at: http://www.myvue.com/olympics
The Magdalen Arms is showing the opening ceremony on a big TV, and probably could be persauded to show more events by popular request!
Blenheim Palace is showing every day of coverage from 28th July - 12th August 10am to 10pm (last admission 5pm) on an outdoor screen. Admission is included in the standard tickets for the house or gardens, with special reduced-price tickets for admission at 5pm. Full details here.