Alphanumeric - If you have a style similar to the plastic ones on their website, you can do it. In fact, if you ever see a pair of frames that you like, Oakley or not, you can always take them to your local optician who can then pop out the lenses, put the frame in their edger and make a new set of lenses in less than 5 minutes. This is provided you don't have some really weird prescription (like a really high minus) that the lenses would be too thick to fit in the frame without a bevel and edge roll which is pretty expensive.
I used to make glasses some time ago. Wholesale, the lenses are incredibly cheap. I have a -1.25 and I used to get plastic lenses wholesale for $30. Most optometrists offices have at least a 300-400% markup. That's why it gets so expensive.