Until when can you choose to get just BA, not Masters at Oxford?

Assuming you mean for e.g. MMath vs BA or similar, then probably in second year. It might vary depending on department though.

Yeah i meant MSci Experimental Psychology vs BA. So generally before the end of the second year? That’s a bit too early, no? How do you make a decision? I thought I would make a decision based on the small scale research project in the third year.

I think it's hard to tell for that course because it looks like they've only just added it. For most courses outside of Oxford and Cambridge with a 4 year vs 3 year version though, it's typical to make a commitment by the end of second year as the third year structure and mark scheme tends to be different dependent which version of the course you are doing elsewhere.

That said I believe at Cambridge for maths since progression to the MMath year is contingent on getting a 1st in third year, they don't actually know if they will be continuing or graduating until the summer between third and fourth year. So there may be less consistency at Oxbridge in general.

Oh wow cam mmath sounds tough! How are you supposed to make your future plan if you don’t know what’s gonna happen until the summer? That’s bit harsh.

Not really sure - although I'm guessing students probably have a general idea of their performance and whether they are likely to continue or not? Looking into it a bit more it looks like the 1st class requirement is just for automatic progression and those without a 1st can ask their college to recommend them to progress.

In any event, generally I would suggest just planning to do the four year course unless something changes in the process to make you strongly prefer not to do it in the first couple years. I would imagine by the end of second year you will have a good idea of your academic trajectory and plans anyway.