Mike Lyle wrote:

The OP wasn't talking of the British menace, the Pink Oxalis: sorry.
_O. corniculata_, though, isn't a N.Am species according to my source,
but a mainland European one, introduced in Britain and Ireland.

The origin is unknown, Oxalis corniculata variant atropurpurea is spread
all over the world now. Some sources say that Africa is the real origin.
All I can say, that noone of my family saw it in our region before
several years - and now it omnipresent. It's not native in US either
(Whitson, T.D. (Ed.) et al. 1996. Weeds of the West.).