Linseed oil is made from the pressings of the dried, ripe seeds of the flax plant that are then exposed to a solvent extraction process. In its untreated form, it's used as a nutritional supplement that is said to ease constipation and lower cholesterol. You can dispose of it by any responsible means in this form. Boiled linseed oil is a woodwork finish and an art material and can be dangerous if disposed of incorrectly.
Put any excess boiled linseed oil that you want to dispose of to one side. Do not pour it down a sink or drain. Linseed oil cures hard and may block drains as a consequence. Do not cover the oil and make sure it is in a well-ventilated area where it will dry quickly.
Allow the linseed oil to cure for 24 hours or until it has hardened.
Throw the cured oil away with your general trash.
Place any boiled linseed soaked rags in the metal container. Linseed oil dries through a process of oxidation that produces heat. This could cause the rags to spontaneously ignite.
Seal the metal container until you're ready to dispose of the rags.
Incinerate the rags.