The Arctic Ocean’s Canada Basin (CB) has seen significant changes in ocean properties in the past two decades. A prominent change has been a warming of the Pacific Summer Water (PSW) layer in the central CB. The corresponding change in dissolved oxygen (O2) is analyzed here to provide additional insight into PSW physics and biology, pathways, and evolution. O2 observations are analyzed between 2003 and 2021 from the Joint Ocean Ice Study/Beaufort Gyre Observing System (JOIS/BGOS) field program, which samples CB hydrographic and biogeochemical properties. In the central CB, warming of the PSW layer over 2003– 2021 has been accompanied by O2 decreases over this time in the layer. Nutrients and other biogeochemical properties are analyzed to quantify the combined influences of both physical changes and biological changes on the evolution of O2 concentrations in the CB PSW. In the upper portion of the PSW, O2 decreases can be entirely accounted for by surface warming (and corresponding decrease in O2 solubility) of its source waters in the Chukchi Sea region. In the deeper portion of the PSW layer, the observed O2 changes are larger, and are accounted for by a combination of the decreased solubility effect due to warming, and increased organic matter breakdown in warmer waters. Decreasing O2 in a warming Arctic Ocean is consonant with O2 trends in the warming global oceans, and highlights the need for continued observations and analyses.