It's only January of year 12 - I'd suggest you go talk to the maths teacher on Monday, explain the situation, and see if you can join the class. Yes, four subjects will be tough especially as you have several months' work to catch up with (and they will probably insist you don't drop one of the other subjects yet), but if you're a viable Oxford candidate, ability-wise, it should be doable. Another option is to carry on with your three subjects and plan to apply as a post A level candidate with your three achieved grades, while taking maths in one year.
If you've realised economics is what you want to do, limiting your options by not having maths is daft. If you didn't take maths because it's really not for you, then it's important to look into how mathematical economics at university would be. It's possible to really like a subject qualitatively but find it horrific once you start trying to do quantitative stuff.