a) Contrary to your assertion, there aren't many spare 165s around and the fleet is fully utilized during morning and afternoon peaks.
b) Tackley and Heyford are pretty small places and unlikely to generate a great deal of traffic
c) King's Sutton is already served by Chiltern between Banbury and Marylebone
d) I suspect the existing two-hourly service runs at a heavy loss in the first place

All of which makes me think that this idea has absolutely no chance of flying whatsoever. Sorry.

No grip on reality.....

There's a very active user group for that part of the Cherwell Valley which you'd have discovered as we're listed on the FGW^▸ (First Great Western) website. It has done a lot of research on passenger counts / numbers and two-hourly is about right commercially. Off-peak fares have even been negituated on the last peak train from the Valley stations too in recognition of the poor service.

Other than slightly more suitable counter-peak timings, there isn't much more the user group can realistically achieve.

Of course, if money were mo object, XC^▸ (Cross Country Trains (franchise)) services wouldn't run every 30 minutes and freight paths would be all at night, thus allowing a 3-times an hour service so no one would need to use their car to reach Oxford or Banbury. But I've just awoken & I think you need to too.