I’m going to post this anonymously as I’d rather not identify myself on this particular occasion.
As far as I’m aware, Oxford would demand for French to be taken as a higher-level subject in the IB, or as an A-Level (they don’t offer it as an ab-initio course, unlike Cambridge, Durham, and most other unis). It’s unlikely they’d accept a DELF score but there’s no harm in asking - though my gut instinct tells me no since it’s nowhere near the same intensity and considering it would be unfair on other candidates. They have a list of accepted qualifications published on their website. If your school isn’t offering French as an HL, you’ll have to take the A-Level, which one can prepare for quite easily (loads of resources out there!)
As MFL courses are also heavily literature oriented, the HL in English would go down quite well on consideration. You’ll also have to take the MLAT test which determines shortlisting for interview by testing your language skills. About 75% of applicants will get an interview, so while you don’t need to “ace” the test in comparison with tests such as the TSA/TMUA/etc, you need to take it seriously.
I didn’t apply for English (dropped the A-Level very quickly, didn’t much fancy the idea of tediously studying Milton’s Paradise Lost) but I know the English course is very competitive indeed, far more so than MFL which has one of the highest success rates at Oxford. You’ll need to ace the ELAT.
It’s also worth mentioning that I’ve heard in passing that joint honours in English/MFL is one of the most competitive courses, and many students will end up just being offered one or the other, usually the MFL component.
And here’s where I’ll remind you about the realities of the admissions process, especially given you’re commenting that you “plan to read English and French at Oxford” as if this is a definite and secured route for you. Your post comes across as very confident (I wouldn’t say arrogant though), and I must warn you to temper your expectations. Oxford has simply more good applicants than they have places. You can smash the admissions test, get good scores and appraisal from the tutors, and you can still miss the mark and not be offered a place. Yes, you have to excel in your subject, with more than a “fascination of the beauty of the French language” I must add (please, for the love of god don’t drop this in your personal statement, nor the fact that French is a maternal language….), and even then you’re still subject to the luck of the draw, what extracts you receive in interview, what questions are asked of you, etc. Simply put, nothing is a dead cert and you can end up sorely disappointed. Don’t let this put you off though. I say this as someone rejected this year with far above average MLAT scores for those accepted, who most believed would get in but didn’t - and I was (and to some extent still am) devastated.
On a final note, take everything I say with a pinch of salt. I’m just a random stranger on the internet and the best point of contact will be the University or your teachers/UCAS advisor. Pick up what you feel is useful from my comment, and leave the rest for all I care. Though please feel free to ask any more questions.