Hi! I'm a first year studying Spanish and beginner's Portuguese at Oxford

I do believe you could have a combination such as Philosophy + a language from scratch, but since French and Spanish are some of the bigger departments under the faculty I don't think they offer it ab initio, which is a shame as I was interested in French but didn't take the A Level </3
Though as I mentioned, beginners language + non-language combo is definitely possible, I have friends doing similar.
But to reassure you, if it is something you would pursue independently, there is a language centre at the uni in which you can enrol onto courses separately from your degree, and are sometimes partly funded depending on your course. I'm not sure if this awards any qualifications/certificates or if you are encouraged to take external tests to get proficiency certification (A1, A2 etc).
Someone I'm doing beginners Portuguese with definitely has a more advanced level, but without qualifications I think you have to apply as a beginner. When applying for beginners' languages, you take the LAT section of the MLAT (+the section for any A Level languages) so I would definitely recommend looking into that and practicing as much as poss! I would say that is the best way to improve on this section as it is quite tricky at first glance.
Regarding interviews, I did have them separately for my languages. I can't say this is the case for everyone but it seems most likely. For beginners' languages it can seem strange to be interviewed on something you've not studied at a qualification level, but it is accommodated to assess your skills and interests in a different way and you are not expected to be fluent! lol
I would definitely recommend having a look at the other beginner language options, as well as all of the combinations available alongside your 'main' subject - there are lots to choose from. One last thing, I'm not sure if the university would consider other qualifications such as A1/2, B1/2 etc., which you can get from DELE, for entry onto Spanish but you can always email the department with any questions
If you have any other questions lmk!