Well , if it helps anyone, Ds who was for clarity , independent school , had the advantage of the school knowing all the dates for admissions etc - i.e. all the admin. This helps from the school - even though available from website - because e.g. school puts in the PS and there are deadlines for references etc. Not hard - just making sure in on time. School was acquainted with the Oxbridge process - so no sense of "not for you" but also , interestingly , would try to discuss with those they did not think it would suit other universities . Discussion only , obviously no school can tell a pupil where they should go.
So I understand the outreach views but the basic timetable is available on line.
DS's PS was written just as a general PS. In fact the one he thought it let him down on was LSE . His wasn't tailored to Oxford. But - no big paragraphs about extra curricular things , it was just about the subject and then 2 or 3 lines at the end about other stuff. It worked for Oxford UCL Exeter, Kings , not LSE ( but to be fair to them their advice on PS is very clear and available on their website ( well , was when DS applied , he either didn't read it , messed up or decided too far from general PS) Probably other reasons why he was not offered a place there but - different story.
I hope I am not being too naive in saying that DS didn't do anything specific to bolster up his PS in his subject . He just has always been interested in history & therefore naturally read stuff , watched documentaries , went to things etc. So no specific classes or anything. I do realise this will not be the same for all subjects. At interview from what I can glean ( and DS is not one for post mortems ) he was able to talk credibly way off the syllabus from school. - that - decent but not stellar GCSEs , and a good mark in the Aptitude test seemed to do it for him.
I was grateful to a poster here who put the aptitude score into context , but really ( for DS's subject) knowing the deadlines for applications , ( so UCAS form in earlier , application for Aptitude test, dates and where to take them - usually at school , is really pretty much the only difference between ( in DS's day ) Oxford and other Universities. All on website.
Ds has a very great friend whose school actually said her Oxford is not for the likes of you. Well , as DS said ( he knows her ) that was a red rag to a bull and she looked up everything. Ended up with a higher final mark than Ds ( whose school was encouraging ) - Ha ha - good for her. 