Well the word on the street here is that Chiltern's new service doesnt fully start until mid 2016, so from 2015 onwards, Most units will be subleased to TPE and the cascade of units will be phased. Its not great for customer but TPE will have to run units individually instead of in multiple to make up the shortfall.
The difference between the interim Chiltern service and the full service is only slight, they are supposedly running their 2 tph Oxford services as far as Oxford (Water Eaton) Parkway, rather than over the last couple of miles into Oxford station itself.
It seems to me therefore, that any savings in rolling stock requirements will probably be negligible, especially if they use the planned paths between Marylebone and Oxford Parkway - i.e. the planned dwell time at Oxford will simply be increased by a few minutes and taken at Parkway.
Edited to add, the draft timetables show a running time of an hour between Marylebone and Oxford Parkway, and about an extra 7 minutes on to Oxford. As they wouldn't normally operate a timetable with no margin for delays, it seems to me they'd need about 3 hours for each return diagram to Oxford Parkway, and 3 1/2 hours for a return diagram into Oxford itself. Lets assume they are 2 unit trains, then I reckon they'll either need either 12 or 14 units on the route - so the shortened route saves 2 units only.
However, the Oxford services are in the main extensions/diversions of existing services that are already running - so most of the stock to be used already exists - but there is a parallel project to increase the length of trains on the Marylebone to Banbury route as well as introduce the Oxford services. All in all, I think it is just a convenience to describe the ex-TPE stock as being needed 'for Oxford'. It is also needed for general strengthening on the existing route - and it is this latter project that is probably the easier to defer.