As they've said before. TRF is supposed to be a good resource - I'm still to join - looks like now is the time to do it. North Oxon is quite rubbish for BOATs and trails in general - at least official ones.
I know of a few short byways, but that's about it. ABR has got some nice routes too, previous issue had something around Stratford, unless my memory fails. Northamptonshire has got a lot more byways to play on, sadly I know very little about them.
TL;DR: There's places out there, finding them is difficult.
Good luck!