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. Within minutes to hours of hypoxic exposure, the lower lip swells with proliferating vascular tissue into a protruding flange that helps channel the thin, better-oxygenated surface film across the gills, a form of aquatic surface respiration. The swelling regresses when oxygen returns. Laboratory studies at Brazilian and North American institutions have documented survival of severe hypoxia for hours by these combined means.
The oscar's tolerance is not unlimited, and it is not an air breather. How long individuals can persist under full anoxia at floodplain temperatures, and how frequently they face it in the wild, is not well quantified.