There are many different types of Oxalis which includes Soursob and Wood Sorrel, it is a major nuisance in lawns and should be treated as soon and as early as possible.
Oxalis is identified by it's trifoliate leaf system with each leaf having a heart shape. Oxalis is usually topped off by a yellow or white flower with three triangular petrels which sits on top of a short stem.
Soursob ia easily identified by it's longer stemmed yellow flowers, the stem of which used to be picked and chewed on for their sour taste by many kids in years past.
Oxalis is considered such a problem weed for the reasons that it can be very difficult to kill, and that it spreads very easily in a variety of different ways. It's root system spreads very widely and as it does it also sets down multiple corms into the soil to begin new plants, and to more easily recover from attempts to kill it.
A full article on killing Oxalis can be found here.
Oxalis is treated with a selective herbicide which can be purchased at the local lawn care and garden store. It is most usually sold in concentrated form which will need to be diluted with water and applied with a Handy Sprayer.
Oxalis is a very difficult weed to kill so results should be monitored after the application of the herbicide. Check the weeds for signs of resistance to the herbicide and signs of life. Re-spray the lawn after two weeks if necessary.
The two week break between herbicide applications allows the lawn to recover and regain it's strength from the earlier application, as well as allowing the Oxalis enough time to react to the poison. Repeat a third time if necessary.
Always check herbicide labels before purchase to ensure the product you are buing is safe to use on Buffalo grass.
The main herbicide used to kill Oxalis can also kill Buffalo grass, so a special herbicide which has been specifically formulated for Buffalo lawns will need to be found and purchased.
If in doubt, treat a small area of the Buffalo lawn first and allow two weeks to test for adverse reactions to the turf, if all is OK - then continue to treat the rest of the lawn.