Absolutely no need to worry about that right now when you have GCSEs so soon. Focus on them and in your spare time, relax and recharge. You sound like you’re doing good so keep it up. Once you’ve done your IGCSEs, over the summer you can finalise your a level or IB etc choices, and if you wish you can start to have a look at the content and prepare for next term. More importantly this is when you can do extracurriculars that ACTUALLY interest you. What’s something you enjoy in maths? Pick a topic and do some reading on it. All this should be stuff you acc find interesting. Then whenever you have spare time prepare by working on your problem solving skills needed for entrance exams. Do a question now and then from step or pat on something you have covered. This can slowly build up your confidence. Not too sure about maths for Oxford but the exams usually change every couple years slightly in format. This isn’t anything to worry about you’ll get enough info near the time so as long as you are working on your thinking and problem solving skills you are good to go. Also these exams will cover things you have no clue about at the moment, don’t waste time right now teaching yourself it will come together later. Any parts on the entry test not in your syllabus you can teach yourself in Y12, but I doubt it will be anything significant if you do A Level Maths. And highly unlikely if you will take further maths.
Do not stress over Oxford entry right now in Y11, it’s great you’re thinking about it but really, the fact you are asking this shows you care enough. Remember even the most top students can get rejected, so it’s also a bit of luck when you apply. This shouldn’t discourage you but keep in mind that Oxbridge is most certainly not everything . For now don’t think about all this prep or skills practice as just for Oxford, just for your future in general and any mathsy degree you choose to pursue.