Life happens. From bike oil to olive oil, it splashes on to us and seems to never come off.
The first thing to do is a bit of spot cleaning with Nikwax TX Direct (or any pure soap) and cold water. Use a sponge or clean cloth to rub the area affected. Then follow the washing instructions on the garment.
Don't leave to soak in this for too long. If you can't do a full wash straight away, rinse or wipe off the detergent until you can wash your item.