Marine organic matter varies widely in elemental composition (C:N:P), and this variability is embedded in the coupled cycling of carbon, nutrients and oxygen. Yet the interacting controls on this variability remain poorly understood. Here we use a global Earth system model with flexible phytoplankton stoichiometry to determine how physiological, ecological and biogeochemical processes shape organic matter composition in the euphotic zone. Across a large ensemble of observationally plausible ecosystem states, N:P of organic matter in the euphotic zone covaries primarily with the global nitrate inventory, consistent with established theories of stoichiometric regulation of the marine fixed-N reservoir. In contrast, C:P of organic matter in the euphotic zone is strongly negatively associated with the ocean oxygen inventory and positively associated with the magnitude of both denitrification and N₂ fixation. Results of parameter sensitivity studies help explain this contrast. N:P and nitrate inventory are shaped mainly by cellular stoichiometric requirements, whereas C:P integrates these quota constraints of organic matter composition with recycling-export and trophic-routing controls on organic matter transfer. These transfer controls strongly influence oxygen consumption and the major fixed-N fluxes, and their overlapping, directionally consistent effects help explain the close covariation among C:P, oxygen consumption, and fixed-N fluxes. These results reveal an emergent coupling between organic matter C:P, ocean oxygenation and the major source and sink fluxes of the fixed-N cycle, extending the interpretation of marine stoichiometry beyond nutrient-inventory regulation.