In an oil storage tank having an interior oil storage chamber and a tank opening in one of the boundary walls, a heating assembly is supported to extend into the interior oil storage chamber from the tank opening. The assembly includes a casing of walls surrounding with a casing mounting flange mountable in sealing engagement relative to a tank mounting flange about the full perimeter of the tank opening such that the casing protrudes into the interior oil storage chamber of the oil storage tank. The heating chamber is suitable sized and configured to operably receive a catalytic heater therein. The heating chamber further accommodates entry of combustion air and combustion fuel into the heating chamber and exhaust of emissions therefrom.