Do you mean you're studying AS further maths - or in Year 12. There aren't many students at Oxford without any further mathematics at all - you can apply, but I just want you to be aware of the challenge. Have you asked your school/college about learning further maths through AMSP?
Re applying - the main criterion should be how you're finding the MAT. Also, the deadline for registering for the MAT is the end of this week (Sept 29).