Oil stains on clothing from salad oil are unfortunately common. Salad oil can drip from a fork or lettuce leaf onto shirts, blouses and pants with ease. Fortunately, such stains are also easy to treat. As with all stains, the sooner the spot is addressed, the better.
Blot the stain with a napkin. Do not rub.
Saturate the stain with a commercial laundry spot remover such as Zout or Out Spot. Let stand for 10 minutes.
Wash with a laundry detergent, preferably one containing liquid enzymes. Use the hottest water suitable for the fabric in question.
If the stain persists, treat with a spot remover again and re-launder.
You may also pre-treat the stain with a commercial spot stick. Dishwashing soap also makes a good stain pretreatment in a pinch.
If you are using anything other than commercial laundry spot remover as a pretreatment, check for colorfastness first.