Lawn weed control
That is one of the weedy Oxalis species (not our native woodland Oxalis) which is difficult to control. It has shallow spreading perennial roots and can shoot seeds over 15 feet!
In areas where there is no lawn and you can spray carefully, glyphosate (Roundup and other trade names) will control it but can also damage any other plant you get the herbicide on. In lawns, there is no great choice. Lawn herbicides that contain "dicamba" as one of the active ingredients works best. Read carefully and follow all product label instructions.
Since Oxalis can recover from seed, control strategies should include keeping the lawn as vigorous as a possible (proper fertilizing and watering) and possibly going to a higher mowing height depending on the grass species you have. Feel free to email me directly if you have more questions: chip.bubl@oregonstate, You can also reach me by phone at 503 841-2200. Chip Bubl OSU Extension Agent/Columbia County