I don't know of any advantage of planting Oxalis but clover (Trifolium), a different genus, is often sown as a green manure. It would be sown on bare ground in summer and then dug in the following spring. Unless there is great benifit that I am not aware of, I would be wary of Oxalis. Some species can be very invasive and difficult to get rid of.

The very word Oxalis puts fear in me, I know someone who's garden is absolutely covered in it.

Oxalis tuberosa or Oca if you are from Chile is grown like a potato with a good crop of edible tubers on its roots. You can get these from a few suppliers.

Interesting article about growing it here in the uk

http://www.wildchicken.com/nature/garden/ga018_growing_oca.htm

I think I will still keep my oxalis confined to pots and troughs!