Walnut oil is pure fat and doesn't contain any FODMAPs.
Note that fat affects gut motility and might trigger IBS symptoms in some persons.
Walnut oil is an oil extracted from walnuts, Juglans regia. The oil contains polyunsaturated fatty acids, monounsaturated fatty acids, and saturated fats.'
Walnut oil is composed largely of polyunsaturated fatty acids (72% of total fats), particularly alpha-linolenic acid (14%) and linoleic acid (58%), oleic acid (13%), and saturated fats (9%).
Walnut oil is edible and is generally used less than other oils in food preparation, often due to high pricing. It is light-colored and delicate in flavor and scent, with a nutty quality. Although chefs sometimes use walnut oil for pan-frying, most avoid walnut oil for high-temperature cooking because heating tends to reduce the oil's flavor and produce a slight bitterness. Walnut oil is preferred in cold dishes such as salad dressings.