DS1 started at Oxford last week (Medicine), so I hope the following is up to-date and relevant.
Obviously, excellent exam results are needed, minimum of AAA at A-level/AAAAA at Scottish higher (+ typically AA at Scottish Advanced Higher, as per my DS1's offer). GCSE would be expected to have a good handful of A*.
DS1 was (Scottish) state schooled, did Duke of Edin silver and gold, had volunteer job 1 day per week for just over a year when his application was submitted (Oxbridge apps go in early, Mid Oct deadline). Did Medlink the year before he applied, 4 day residential at which there was an Oxbridge delegation and lectures - that was for potential medical students, but I think the same kind of residential courses exist for other subject areas....
Your DS will be applying in 12 months - time to get some CV enhancing activities on the go, right now! NO, Oxford don't care about your charity work and your Dof E, but they do care about the fact that a packed life shows you can handle an academic workload and still fit in other things. BUT, the other Unis he applies to will care about these things, so it won't be time/money wasted.
It is VERY important for DS to be able to show that he has read in depth about his subject, way beyond the school curriculum, or has undertaken work or self-motivated projects - Oxford interviews take place over 2 days and they will be looking for evidence of great enthusiasm for his chosen subject.
I'd suggest subscribing right now to the Economist and making sure DS reads it! Any interest in computing/programming (NOT computer games!), should be encouraged - high finance is all about computer programs these days....