Place a paper towel underneath the raincoat, directly under where the stain is. Cover the stain with a generous layer of cornstarch. You absolutely must saturate the oil stain in cornstarch.
Allow the cornstarch to soak up the oil for at least 20 minutes. As the cornstarch is working, cover the top of the stain with another paper towel.
Place an iron, set to medium heat, over the top paper towel. Run the iron back and forth a few times over the paper towel.
Remove the paper towel and brush the oil stain with a soft dry brush, such as a shoe brush or a brush for suede. This will remove all the corn starch and much of the oil also comes out. Repeat this entire procedure at least one more time to remove the oil stain completely from your white raincoat.