In 2015 researchers at the NOAA Southwest Fisheries Science Center reported that the opah has a network of blood vessels in its gill tissue in which warm blood leaving the body core runs alongside cold blood returning from the gills. Heat passes from one to the other, so the blood arriving back at the heart has already been rewarmed.
The heat itself comes from the constantly flapping pectoral muscles, which are insulated by a layer of fat. Measurements on fish caught off California found muscle temperatures around five degrees Celsius above ambient water, sustained while the fish stayed at depth.
The distinction from tunas matters. Tunas and lamnid sharks are regionally endothermic, warming swimming muscle, eyes or viscera while the blood returning through the gills loses its heat. The opah's counter-current exchanger is positioned in the gills themselves, which is why the whole-body claim was novel rather than another instance of a known pattern.