Oxford undergraduate economics has definitely got more mathematically rigorous in recent years (the emphasis in exams has shifted from essays to more problem-based questions, and probability and stats is now a third of the first year course), but the maths element is probably still not that challenging if you’re highly mathematically able, at least until third year.
The maths in the core micro and macro papers in first and second year is definitely not very difficult at all; largely just pretty simple differentiation and some infinite series (the hardest stuff is probably constrained optimisation/Lagrangians, which, while you won’t be familiar with it from school, is mostly just mechanical once you get the hang of it). As mentioned, there’s a probability and stats element to the first year course, but this is not very advanced; it’s basically all stuff you’ll be familiar with if you did the stats papers in further maths A level, and is widely viewed as providing the easiest marks on the prelims paper.
The first year probability and stats content is extended considerably in the second year quantitative economics paper, and then in third year, there are three ‘technical’ option papers: econometrics, micro analysis, and game theory. So, if you want to, you can certainly pick a pretty mathematically-focused set of finals papers.