I imagine for most people it's not a food at all, but some Oxalis species have a long history of culinary and medicinal use. One is wood sorrel (Oxalis acetosella).
It has small, pretty, lime-green leaves and pinkish flowers.
Raw, the leaves have a sharp, sour, slightly lemony taste. They can be used to flavour soups and salads. Because of the plant's high oxalic acid levels you don't want to eat too much, but general consensus seems to be that in small amounts, it's fine. (Unless you have gout, kidney troubles or rheumatoid arthritis and are avoiding all high-oxalic-acid foods.)
Wood sorrel is often paired with fish. Appropriate I reckon, since most ocean-caught fish should be a Sometimes (If Ever) Food too - given the world's declining fish stocks.
The other night we had tarakihi baked in a herb butter I made from wood sorrel and onion weed. We were in a hurry to go somewhere so I ended up putting it under the grill for the last couple of minutes to speed the whole process up.
The combination of onionweed and wood sorrel was lovely. Definitely going to make it again, and next time just oven grill it from start to finish.