And that's fine, they just need to use the P&R. Which is good, but could probably be improved further, e.g. late evening frequencies are a bit poor and it struggles to cope with the large numbers at once from a theatre performance.
The remainder who will still drive if there was a tram could use a P&R with a large ZEZ, I agree.
However, you won't handle both the urban area and country passengers with P&R alone, no matter how many buses you use to transport people from the P&R, unless you have some sort of (at minimum) guided busway between most of the P&Rs and the centre (at which point, you may as well invest in a proper regional light rail/tramway network).
There's too much local congestion within Oxford from local residents and narrow arterial roads to run a sort of Glider-style BRT into the centre using continuous bus lanes, and the property demolition you would need to widen those roads is completely untenable.
You also have the issue of the P&R capacity being difficult to expand at many sites due to floodplain/Green Belt land (the Thames Valley area has a great example of a P&R that was built on floodplain and spent more time as a ersatz marina than public transport hub - the old Winnersh Triangle P&R by the Showcase Cinema) or adjacent residential areas.
This is why you need any dedicated public transport corridor separate from the road (whether it be guided busway, tram, railway etc.) to extend out at least part of the way beyond the city boundaries towards Thame/Abingdon/Witney, to stop the more restricted P&Rs getting clogged up with cars from the larger suburban centres in the metro areas.