There can be no doubt that it is preferred for applicants to have maths at A level. However, Oxford are also aware that many candidates are poorly advised. If you were to mention that you didn't take maths because you were told that it wasn't necessary, then I expect that they would understand this. If you then took maths up at AS level in upper 6th, I expect the tutors would be able to overlook the fact you don't have the full a-level.