Chiltern have posted the new timetable valid from 13th December through 16th May.
There are no significant changes (AFICT) for Bicester, but there is one damning one (IMHO) for Islip. No doubt in an attempt to spread things out more evenly, but without any passengers in mind, and without any consultation, they have removed the 09.02 from Islip weekdays into Oxford, exchanging it for a 10.02. So people dropping off children before going to work, those who start a little later and those who just wanted to avoid the early morning crush are politely stuffed. (Please excuse me if you differ, but I know I speak for some, at least.)
The new complete timetable may be found here.
While this is my concise version for Islip folk. (Subtract 6mins from Oxford time for the parkway station. Add a large galollup for the slow bus link which replaces the train on the last leg (a lot less perhaps if you take a bike instead).)