Details can be found
here on the Oxford Bus Company website.
Essentially it boils down to tickets for journeys that would normally require use of the line if it was being served by trains.
The examples of tickets they'll accept include:
- Oxford to/from Bicester Village (or Oxford to/from any Chiltern station other than Marylebone)
- Reading to/from Oxford Parkway (routed not via London)
- Aylesbury to/from Oxford (routed not via London)
- London to Islip (unless routed via High Wycombe)
And tickets they won't accept include:
- Tickets between London and Oxford (or London and Oxford Parkway)
- Tickets between London and Bicester Village (routed via High Wycombe or "Chiltern Only")
- Tickets between London and Kings Sutton/Banbury
- Tickets between London and Islip (routed via High Wycombe)
At present, fares between Oxford Parkway and Oxford are set at the same level as the bus fare (i.e. £1.80 Single or £2.80 Day Return) so they can be sold in the same transaction as a ticket to Oxford or Oxford Parkway, and Oxford PlusBus is also valid on the route (£4.00 Daily, £15 Weekly, £45 Monthly, £120 Quarterly and £390 Annually).
Consensus of opinion is that All Line Rovers (and BritRail/InterRail passes and such like)
SHOULD be accepted, but they'd be far and few between, so it's entirely plausible that Drivers aren't aware and you may be better off just buying a ticket and seeking a refund through Customer Services (though whether it'd be OBC or Chiltern remains to be seen).