Are you currently living in Britain and preparing for BPhO? (You don't have to answer this, but it would make it easier help wise to know which country's olympiad you're preparing for and whether you're applying to Oxford as a foreign student or not - I'm going to assume you are from Britain for now)
There is no guarantee that an Olympiad will get you into Oxford University (from what I've heard from many people who've applied, and got in, or not got in, to Oxford for physics), not even near. Your main motivation for doing Olympiads should be
enjoying them: I cannot stress enough how much of a difference for preparation this has made for me!!
In terms of help, you've asked for it in the right place, here is my suggestion as someone also doing a lot for BAAO/BPhO:
- Books with lots of questions (practice practice practice!) My favourite is Young and Freedman's University Physics which you can get for quite cheap second hand, but another good option is Halliday, Resnik and Krane's Fundamentals of Physics. Both of them explain well and have an absolute wealth of questions. If you're looking to do astronomy, Salvatti's Fundamentals of Astronomy is GREAT.
- Knowing the exam - knowing how long it is, how many marks it is, what sort of questions come up. For example in BAAO R1, the paper 2 always has an observational astronomy question which is the least answered question but has the highest average score and is often the least time consuming too.
- I've personally found ZPhysics's videos, newsletters (you can sign up on the site!) and LinkedIn posts to be quite helpful - other Youtube channels like the official BPhO channel can also be helpful particularly for touching up on weak topics
- Notes and guides - Kevin Zhou's already been mentioned but there are other great guides like this one which are worth going through.
- Past papers. This and the syllabus honestly might be the single best resource you have. Do them, and go through them well. If you run out, do the past papers of other countries, e.g. try the F=ma. Keep practicing.
But also, speaking of Oxford, if you're interested in applying they have a great
course about preparing for the PAT which you might find handy!
Hope this helps! Also, a lot of these resources were actually suggested to me by
@Muu9 so shoutout to them :)