I'll visiting Oxford for the day (maybe four our five hours), on a weekday in the last week of July. I'll be driving a rented car, coming from London, on my way to the North of Britain. Where would it be most convenient to leave the car while visiting the main highlights? (university campus, library, etc)
Your best bet for Oxford is to not have a car! Really...
There are a handful of parking sites in the city centre, but they're very expensive, and often full. Especially at the moment with all the building works for the expanded Westgate Centre, which means that the main shoppers carpark is a building site.
If you really must drive in, and don't mind traffic jams, loads of cyclists everywhere, and a fair chance of getting a fine in the post after you get confused and drive down a bus+taxi+cyclist only street (of which there are many), then:
- From the West, there's temporary shoppers parking at the Ice Rink on Oxpens (10 mins walk from the centre) that isn't always full, and the Worcester Street carkpark (5 mins from the centre) that is full most of the day
- From the North, there is parking on St Giles (central) but that's almost always full, and Gloucester Green car park (ditto), otherwise you can slog in traffic round to the ice rink
- From the East, there's basically nothing - you have no hope most of the day getting a space on Broad Street for example
- From the south, the ice rink
Oh, and those are all council run, expect to spend £3-£4/hour. Full details on the city council website
There is a handful (and by that I mean fewer than 10) of free spaces on streets near the centre, all 1 hour or 30 minute limits. Unless you're happy with a 25+ minute walk (map of permit parking zones - you need to go out beyond those), you can't park for free
The recommended option is to use the Park and Rides. These are big car parks around the edge of the city, with regular quick buses taking you right into the city centre. There is at least one on every major route into the city, sometimes more than one. No need to worry about narrow roads, no parking, fines for using bus lanes etc, easy!
Otherwise, the railway station is a few minutes walk from the city centre, the bus/coach station is even closer. Why not take a bus / coach / train to the city instead?
About your only central free parking spots are for bicycles, motorbikes and canal barges
Oh, and one other thing. You say visiting the main highlights? (university campus, library, etc) - You probably need to read up a bit more on Oxford! There's isn't a library, there's several famous ones, and about a hundred in total! Also, Oxford University doesn't have a campus, it has a city. Where other universities might have space between their buildings, Oxford has bits of city, including lots of bits of "boring old city" that are older than some countries...
What counts are "most convenient" would be a value judgement, but I can present some options.
Oxford's traffic and parking is in general pretty bad. July should be a bit quieter, because of summer holidays, but a weekday does mean that workers will be using most available parking space.
Your lowest-stress option is probably to use a park-and-ride facility. With these, you park your car in a large, easy-to-access car park outside of the city, and then get a bus in to the town centre. The prices vary depending on which car park you use - Pear Tree and Thornbridge both mention being accessible from the M40, which fits with your itinerary.
Somewhat unusually, payment for parking and bus is apparently separate in Oxford. You can find more about the bus services (and fares) on the OxfordBus site.
If you wish to park closer to the city centre, it seems that most parking is operated by Oxford council. If you are able to arrive and leave outside of peak commuter times, this might not be too bad an option. You can find out more about the council parks (and rules) on the Oxford parking site. One notable option is to reserve in advance a permit at the Oxpens site, guaranteeing you a space - although it is about 20 minutes walk from the University.
I don't know what the options are with regards to on-street parking, especially on the out-of-centre side of the University, but from what friends and family have told me of driving and parking in Oxford, I'd be surprised if there was much opportunity. Still, you could drive around having a look if you want. Just pay careful attention to signage regard availability and time limitations to avoid a substantial fine.