Oxford degrees are extremely confusing. As stated, an Oxford BA degree, which requires 3 years study, can be converted to an MA after 7 years, simply by paying a fee.
Some undergraduate courses, which last 4 years, lead directly to a Master's degree, such as Master of Mathematics. However, for 7 years after matriculation, the recipient can only wear a Bachelor's gown!
However, some other Oxford Bachelor's degrees (such as Bachelor of Music or Bachelor of Divinity) are actually post-graduate degrees which, in any almost other institution, would normally be Master's degrees!
http://www.answers.com/topic/degrees-of-the-un iversity-of-oxford
Although MA(Oxon) can look impressive after somebody's name, some bodies don't accept it as a 'real' Master's degree:
http://www.nzqa.co.nz/
Chris