METHOD. When an oil mark happens it's important you do not wash or wet the garment, this can lock in the problem. Instead, put a flannel underneath the affected area. Then shake a generous amount of talcum powder over the stain on both sides and let it sit for a couple of hours.
REPEAT AS REQUIRED. Use a clothes brush to brush off all the talcum powder. If the oil mark has not been absorbed, repeat and leave overnight. Please note that if it's an oil mark from food you will need to hand wash the garment afterwards. You'll need to dip the garment into warm or cold water, either in a bucket or in the shower, then allow it to gently drip dry. This avoids the garment watermarking.