Hi there, hope I can shed some light on your questions,
Oxford is unlikely to care about work experience unless it's extremely relevant to the subject field and something you've acquired off your own back, i.e. not through family connections. Even then, most candidates won't have it, so if this isn't you, I wouldn't worry.
I'm not sure if there's an admissions test at Oxford for Neuroscience; however, if there is one, this will form a very important part of shortlisting for an interview, and you need to perform well on it. I can't speak for their department as I have no experience with it. Still, with a high score in the IB diploma and a decent personal statement, you're likely to get an interview as candidates with achieved grades already have proven potential. I can't imagine Neuroscience is as super competitive as other courses to warrant a super selective interview process - but you should check this information for yourself as it's available on their website.
As for super-curricular activities, they want to see wider reading around your subject and perhaps some participation in essay competitions/olympiads; your passion really needs to shine through the personal statement. The PS may form the basis for interview discussion, or they may totally ignore it, but treat it seriously.
The IB is perfectly fine for satisfying the admissions requirement (as is the Abitur if you're also sitting that, as I know that some schools in Germany run a dual programme), and their points requirement broadly matches the expectation from A-Level candidates.
A summer school is a great way to show your interest in the subject if mentioned on a personal statement, and a great way into the subject - but again, this isn't super critical for Oxford, and I'm assuming you've paid lots of money for it (companies like Immerse, etc.) so you won't be getting any "extra points" as it's not something that's really accessible for all candidates.
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