www.Portlandmaps.com has always been a great resource. It just got better. They city has added all of the oil tank permits that they have on file. It is the far right option on the screen on the property tab under “storage tank.”
Just keep in mind that if there is no record of a permit doesn’t mean there wasn’t an oil tank installed. It is another resource, not a definitive answer to if there is or was a tank on the property.
DEQ also has a lot of information on oil tanks.