They vary depending on the variety - some are frost tender, some are moderately hardy, some are very hardy. Oxalis tetraphylla is moderately hardy, meaning it'll be fine in a sheltered spot.

It is a perennial but though I have know the odd plant come through a winter when planted in the ground, they really need bringing indoors unless in a sheltered spot.

Some people say 'unfortunately, yes!'

I'd like to see a winter polish off Oxalis adenophyla, Fractal...

I thought this question was specifically about Oxalis tetraphylla?

Oh, I see, didn't realise you meant just that variety. I think its on tetraphylla, but you can never be sure when people ask from the plant pages, sometimes they mean generally.