Oxalis is not one of the heavyweights in the herb world because of it's high Oxalic Acid content.  Oxalic acid occurs naturally in small amounts in many vegetables and fruits, including rhubarb, spinach, mustard greens, poppy seeds, sweet potatoes, cabbage, pumpkin, strawberries, mangoes, tomatoes, and parsnips, to name a few.  Too much Oxalic Acid binds with other nutrients, making them inaccessible to the body, and the acid itself is strong, causing irritation to tissues.  Eating too much of a plant high in Oxalic Acid can cause digestive problems and kidney problems, and at extremely high doses it can be fatal.  Taken in small quantities is perfectly safe, however, except for people with kidney disease or kidney stones, gout, or rheumatoid arthritis, who should avoid it if at all possible.

Having said that, Oxalis does have some limited medicinal uses for minor complaints.  It's cooling action appears to help reduce the discomfort of fevers taken in a tea made at 1 ounce of herb to 1 pint of water. There is some indication that taken in small doses of no more than 2 fluid ounces, Oxalis can provide some relief from urinary tract infections.  Taken as a gargle it also seems to have some effect on mouth sores or ulcers.  Externally, it appears to be of benefit when made into an ointment for cuts, scrapes, rashes, and skin infections. 

Once again, any remedies made out of Oxalis (or with any herb for that matter) should be taken in MODERATION as per instructions above.