This small perennial is native to the mountains of New Zealand and Tasmania. Probably brought in as a dainty rockery plant, but is now a real pest in the garden as it spreads by seed and creeping stems. As for the other Sorrels the plant contains oxalic acid so has similar medicinal and culinary uses - it is also high in Vitamin C. Some of the herbal remedies it could be used for are, to stem bleeding, urinary tract infections, as a diuretic and an astringent to contract tissues and ease injury. The leaves and flowers add a sharp tange to salads, but only in small quantities.
It prefers open areas, tolerating a range of soil types, spreading in the crevices in paving and covering flowerbeds. The small, five-petaled, yellow flowers are borne singly on a stalk arising from a node. Later a small acorn-shaped seed pod develops.
Uprooting it is relatively easy, but it soon returns from the seed bank it has created over the soil.
Repeated use of Glyphosate should work eventually. A residual agent should prevent germination for a while.

Procumbent Yellow Sorrel (O. corniculata) is a similar, slightly larger plant, but has flowers on clusters

See also Creeping Wood Sorrel, Clover and Wood Sorrel.