British Rental Car

Yes, just make a search and do this:-

During the booking process once you have entered your search criteria, you will see car photos (results). At the top right hand side is a drop down filter, choose "SORT BY DISTANCE”. This will shuffle the available cars based upon nearest, with a small map. The specified distance above the map is the distance from the inserted desired location to the location of the nearest supplier. Click on the little map to expand and the exact address/location will be shown.

I see you searched for Oxford only. Insert "Oxford Train" and choose Oxford Train station from the drop down menu. All the car rental companies are nearby but no longer at the station

Anyway with a large group like you have I would suggest that one of you jumps in a taxi and gets the vehicle and returns to the station. Taxis are plenty at the Station.

Yes, you can return the vehicle anytime using the key drop-box facility.