Unfortunately we do not have a selective herbicide registered to treat  Oxalis pes-caprae (Sour Sob) in tall fescue.

Try 'Yates Zero Weedkiller Super Concentrate' as a spot spray, or paint on with the 'Yates Zero Weeding Brush'. Please use caution when using the weedkiller near other plants/lawn, because everything that it touches could potentially kill. 

Please click on the following links for further info on these products:

'Yates Zero Weedkiller Super Concentrate'- https://www.yates.com.au/product/garden-weed-control/general-garden-weed-control/yates-zero-weedkiller-super-concentrate

'Yates Zero Weeding Brush'- https://www.yates.com.au/product/garden-weed-control/general-garden-weed-control/zero-weeding-brush

We recommend using a piece of cardboard to mask the lawn from impact of Zero while applying to the weeds you wish to control.