Abstract: A blue flame oil burner is provided wherein an oil atomizing nozzle produces sprayed oil droplets as a symetrical hollow cone having an included angle for the outside edge of the oil spray, when the oil is sprayed in still air, in the range 60.degree. to 80.degree., an air swirler with an air pre-swirler both have blades with a helix angle in the range 50.degree. to 65.degree., with the blades of the air swirler adjacent the spraying end of the atomizing nozzle inclined in the direction of the hollow cone at an angle in the range 20.degree. to 55.degree., and a supply tube for delivering air through the air pre-swirler to the air swirler. The air flows along the supply tube and is preswirled in the pre-swirler and then finally swirled in the air swirler so that sprayed oil droplets in the hollow cone are carried by the swirling air, which diffuses with them in such a manner that the oil droplets are evaporated prior to combustion and:I.