Are oil based paints illegal in California?

I don't think the small tubes of oil based paint for artists, which are usually 2 or 4 ounces, are restricted by the v.o.c. numbers regulated by the South Coast Air Quality Management District. If you don't get a MSDS when you buy it, you should not worry about it. Your only real concern should be if you are making quarts or gallons. The issues are the chemicals that out gas when drying. If you really need large quantities of oil paint, oneshot still makes quarts for signpainters.