Wood sorrel (also called Oregon sorrel or redwood sorrel) is another Pacific coast native, a plant of the coast ranges from central California through Washington. It is common from the wet side of the Olympic Peninsula south. The WTU Herbarium lists it as occurring in King County, but I could only find one non-garden-origin specimen in the database, from a Mercer Island address.
Neither WTU herbarium nor the USDA Plants database show it as occurring on the mainland North of King County. It is listed as a “species of special concern” in British Columbia.
The name Oxalis comes from the Latin word for sour. The plant contains oxalic acid, which can be toxic in large quantities. It can combine with calcium or other metals in the body to form kidney stones. Oxalic acid is also the component of rhubarb leaves that makes them toxic.
The US EPA classes oxalic acid as a pesticide, mostly because it is used as a disinfectant in some bathroom cleansers and swimming pool treatment systems. Exposure to concentrated oxalic acid can cause skin irritation and eye damage, but it degrades quickly, and is not considered a threat to wildlife.
In Ethnobotany of Western Washington, Erna Gunther states that the Cowlitz and Makah tribes use the juice of Oxalis as an eye wash. Other tribal uses are as a wash to counteract skin irritations and rheumatism, and as a trail snack.
In the garden, Oxalis oregana is an appealing groundcover with shamrock leaves and pink-tinged white flowers. It tolerates deep shade, exhibiting nyctinasty (folding its leaves in response) when subjected to bright sunlight. It is also rhizomatous, and will spread to cover whatever area it can. (This is a beautiful thing if you want to fill in bare ground between the sword ferns under your rhododendrons; not so much if you have planted it with delicates in your formal border.)
So. The sour leaves and flowers of O.oregana make a wonderful trailside snack or addition to a salad; just don’t overdo it. Plant it somewhere where you won’t be battling its exuberant nature. And, if you find it growing wild (not as a garden escapee) north of Olympia, please tell me!