There are a large amount of elderly houses and farms, still using oil burners as their primary heating source. Some 40-50 years ago, this was THE cheapest source.
Oil furnace systems are normally using a waterpipe distribution, covering the entire building.
As oil nowadays are very expensive, this is now depreciated in favor of other sources. Most oil furnaces are quit easily converted to accept alternatives at a reasonable cost.
To take in considerance when convering a system may be the need of an additional circulation pump. As the operating temperature most likely will be quite lower than before, a self-cirkulation system may experiance problems.
Oil is a non-regeneration source, thus releasing greenhouse gases, making it a less appriciated source.