I hope you are up to the challenge because oxalis is a persistent weed due to the way it spreads from both seed and underground roots. The first tip is to thicken up your lawn so that it can crowd out the oxalis. Water deeply and fertilize the lawn regularly to thicken it up. You can hand pull oxalis before it blooms in the spring and goes to seed, but for a large lawn you will need to resort to a broad leaf weed killer. There are many brands on the market but use one made for lawns and check to make sure it says oxalis on the label when it lists what weeds will be controlled. Another tip is to collect the lawn clippings as the seeds can hide in the grass clippings. It may take several years of using weed killer and thickening the lawn but if you are persistent you can control this pesky weed. Keep growing, Marianne Binetti