Climate-driven reductions in dissolved oxygen (DO) of the global ocean interior (ocean deoxygenation) is leading to expansion of permanent oxygen minimum zones (OMZ) that comprise about 7% of ocean volume. Impacts on marine animal distributions and abundance may be particularly significant for high-oxygen-demand top predators, such as warm-bodied tunas and sharks, by reducing habitat volumes as OMZs expand (habitat compression) and concentrating fish further in surface waters where they become more vulnerable to fisheries. But predictions of how exploited oceanic fish actually respond to OMZ expansions are not based on mechanistic understandings, principally because direct measurements of oxygen tolerances and associated metabolic costs have not been determined. OCEAN DEOXYFISH will bring about a step change in understanding of OMZ impacts on oceanic ecology by applying our existing expertise in animal movement studies and by developing new biologging technologies and in situ physiology for measuring oxygen tolerances and metabolism directly in free-living fish. This will enable major unknowns to be addressed concerning how oceanic fish respond physiologically and behaviourally to hypoxia, the role of OMZs in upper-trophic-level ecology, how oceanic fish habitats change with predicted OMZ expansion, and whether this will increase fish vulnerability to fishing gear. We will achieve objectives through linked field, experimental and modelling studies. By focusing on key processes underlying fish responses to DO in situ, new modelling approaches will establish effects of future warming and OMZ shoaling on fish niches and determine how these shift distributions and alter capture risk by fisheries. The project represents a discipline-spanning approach linking physiology to ecology and oceanography, with wide-ranging outcomes for understanding global biotic responses to warming and ocean deoxygenation with direct relevance to sustainable fisheries and species conservation.