Oleic acid is a monounsaturated omega-9 fatty acid you get from olive oil, avocados, and nuts. People take it to help keep cholesterol levels healthy where they already are and to nourish skin and hair, both by mouth and in topical products.
Most intake comes straight from food. Supplement forms include liquids and softgels, taken with meals so the body can absorb it properly. Whole food sources are far more common than isolated pills.
Adding a lot through supplements means extra calories. Some people notice mild stomach upset. If you take any medications or have health concerns, talk to your doctor.