In flooded forest and blackwater lagoons, decomposition strips oxygen from the water column, especially at night. Oscars respond with a set of measures rather than one. Metabolic rate drops steeply, tissues shift toward anaerobic pathways, and heart rate and activity fall, which extends survival well beyond what routine oxygen demand would permit.
They also use the surface, tilting up to draw the thin, better-oxygenated film across the gills, a behaviour known as aquatic surface respiration. Laboratory work has recorded adults enduring severe hypoxia for at least twenty hours and complete anoxia for several hours at floodplain temperatures. Unlike the tambaqui, an Amazonian characin that grows a swollen, vascularised lower lip within hours of hypoxic exposure, the oscar develops no such structure.
The oscar's tolerance is not unlimited, and it is not an air breather. How often wild oscars actually encounter severe hypoxia, and how the laboratory limits translate to a warm floodplain lagoon, is not well quantified.