As part of the interdisciplinary project S1-INBOX (Western North Pacific Integrated Physical-Biogeochemical Ocean Observation Experiment conducted around the S1 biogeochemical mooring site), we used data from more than 18 floats and a biogeochemical mooring S1 (near 30° N, 145° E) to investigate temporal and spatial changes in the shallow oxygen maximum (SOM) associated with a mesoscale cyclonic eddy. On the northern edge of the cyclonic eddy, patches (linear dimensions of 20–40 km) with relatively high oxygen concentrations were observed around the SOM. The patterns of the oxygen concentrations reflected the fact that changes of the depths of the isopycnal surfaces were caused by small disturbances associated with the eddy structure along the eddy edge. The implication is that nutrient-rich water was supplied by upward isopycnal heaving at the edge of the eddy and contributed to the formation of the high-oxygen patches. As relatively high oxygen concentrations on the same isopycnal surfaces at greater depths were sometimes observed in the region downstream of the high-oxygen patches, we suggest that the patches were advected to the downstream region. The high-oxygen water seemed to extend into the eddy core from its edge. Ageostrophic secondary circulation around the edge of the eddy might have contributed to maintenance of the high oxygen concentrations in the eddy core, and these high oxygen concentrations may have been formed during spin-up of the eddy by heaving of isopycnal surfaces.