There are many cultivars of Oxalis and many are desireable garden plants. One that many people consider a "weed" is a plant that many beneficial insects rely on for survival if the prey they also live on are not present, so destroying Yellow Wood Sorrel can be a bad thing.

Here is a link that might be useful: Yellow Wood Sorrel

Where do you live in SoCal? I promise we won't all show up at once ;-) but SoCal covers Calexico to Santa Barbara including Big Bear. Some things will work in some areas that won't work in others.
What have you tried to get rid of the oxalis?
Can you post a picture of the oxalis?
What kind of grass do you have?
What do you fertilize with and how often?
How often do you water and for how long?
How high do you mow and how often?

In all the discussions about rust here over the years, I have never read the words, "I lost my lawn to rust." I think rust will go away by itself. The worst thing about it is the orange dust on your shoes.