Whether you are going to Oxford or not, here are my personal tips on preparing to study chemistry at university:
Keep reading over summer! Being able to gain concepts from reading textbooks/online is an important skill. If you’re given a reading list, don’t worry about reading it all - I think it’s more about developing the skill of being able to self-teach from reading.
Before uni, try and polish up your maths skills again. Useful topics: differentiation, integration, binomial expansion. If you want to learn extra and haven’t done Further Maths: complex numbers, Taylor (& Maclaurin) Series, and first order differential equations are just a couple of useful topics that you will cover later.
This is more specific to Oxford, but these topics probably generally apply to many other universities also:
Disclaimer: none of this is totally essential, as I did very little of any of this (despite planning to) and have still managed. Instead this is what I wish I had done last summer, which would have been helpful. Also, some people may prefer that you don’t overly self-teach before arriving.
Try and work through Oxford's MPLS Bridging Programme. I can’t remember how much content there is, but getting used to reading and assimilating knowledge by yourself is definitely a useful skill!
You’ll get plenty of practice on the course, but practise differentiation and integration, so that you are at a level where you find them straightforward and quick.
Using online resources/an A-Level textbook, I’d recommend familiarising yourself with the following topics: mechanics, electrostatics, magnetism, light and waves, heat and internal energy, and the behaviour of gases.
Don’t worry if you don’t fully understand what’s going on, but at least having a look at the formulas will mean they don’t look so alien later on!
On a similar note: http://course.chem.ox.ac.uk/physics-for-chemists.aspx is a handy website, as you’ll be covering the electrodynamics and magnetism parts during first year. This is probably something to do if you’re already comfortable with the physics aspects and want to extend your skills/knowledge.
It might be on the Bridging Programme, but get comfortable with the idea of atomic orbitals and hybridisation, particularly if it hasn’t been covered on your course.
MOST IMPORTANTLY: Don’t stress! Sleep lots, eat well and have fun! Relax and enjoy your summer!!