Unlike fuel gas in the previous quiz, an oil burning appliance will need two openings (or ducts) to the exterior. But since this is a simple question and I am only asking for the opening sizes we will not worry about ducts. One opening to bring in outdoor air will have to be located within 12" of the top of the enclosure and the other will have to be within 12" of the bottom of the enclosure. Since the openings will be directly communicating with the outdoors we will need 1 square inch per 4,000 BTUs of appliance input rating PER opening. Therefore, with a 60,000 BTU input we would need 15 square inches (net free air) per opening. So if you used metal grills with a 75% opening rating. Each grill would have to be a minimum of 20 square inches.