Arctic fjords are rapidly responding to climate-driven changes in freshwater input, stratification, and glacial meltwater discharge, yet oxygen cycling and its coupling to particle-mediated biogeochemical processes remain insufficiently constrained. Here we investigate dissolved oxygen (DO) dynamics in Kongsfjorden and Grønfjorden (Svalbard) using DO–δ¹⁸O, O₂/Ar, and a respiration–mixing (RM) model to quantify the contributions of water column respiration (WCR), sedimentary oxygen respiration (SOR), and oxygen-supply-limited aggregate respiration (AGR). Kongsfjorden exhibits strong stratification with a pronounced pycnocline (~75 m), whereas Grønfjorden is well mixed due to stronger tidal exchange and reduced depth. Correspondingly, Kongsfjorden shows marked vertical gradients in DO and DO–δ¹⁸O, with surface supersaturation reflecting dominant photosynthetic production and deep enrichment indicating respiratory oxygen consumption. In contrast, Grønfjorden remains near oxygen saturation throughout the water column, indicating balanced production and consumption under strong mixing. Net oxygen production is positive in Grønfjorden but near balanced or weakly negative at depth in Kongsfjorden. The DO–δ¹⁸O–ln(DO%) relationship is significant only in Kongsfjorden, indicating strong modulation of isotopic signals by stratification and mixing. RM modeling further reveals that AGR dominates oxygen consumption in mid-depth waters (150–300 m), contributing >50% of AOU, while SOR becomes increasingly important near the seafloor. These results highlight the importance of particle-scale diffusion limitation in regulating oxygen consumption and weakening isotope fractionation in stratified fjords. Importantly, oxygen cycling is closely linked to coastal nitrogen transformations. High primary production and particle export enhance organic matter remineralization, which stimulates aerobic respiration and nitrification, while also influencing nitrogen retention versus loss in bottom waters. Aggregates thus act as biogeochemical microreactors coupling carbon degradation with nitrogen cycling under high-productivity Arctic conditions. Overall, oxygen dynamics in Arctic fjords are governed by interactions among stratification, water-mass mixing, and particle-scale respiration. This study emphasizes the critical role of aggregate-mediated processes in regulating coupled oxygen and nitrogen cycling in a rapidly changing Arctic Ocean.