OBK!
The other way round - Jeppesen distribute the Oxford manuals for those not on Oxford courses. The covers say both OATS and Jeppesen, but the manuals were written in-house at Oxford. There is nothing more 'official' about Oxford's manuals than Bristol's, GTS's or ours (BCFT).
Note that manuals written in-house tend to have the advantage, as the copyright does not stop the instructors ammending the manuals as it does with licensed manuals. The instructors know what areas need changing (and I can tell you it's a continuous process!) and can push the changes through quickly. I would say that I have helped students with private study when I was self employed, students of distance learning from both courses, so I have taught from both sets of manuals, and I would not put Oxford's above Bristols. I have met Alex Wittingham (of BGS) and like him, and Bristol have efficient administration.