Oxygen Deficient Zones (ODZs) are key areas of N loss, a process dependent on organic matter. Understanding the sources of organic matter to the ODZ is necessary to predict how biogeochemical cycles will respond to ocean changes. Size fractionated (5-20, 20-53, 53-180, 180-500, >500 µm) particulate organic C and N (POM) concentration and isotopic composition depth profiles from three stations in the offshore Eastern Tropical North Pacific (ETNP) ODZ were used to create previously missing ETNP specific particle size to carbon and nitrogen relationships for models, while gaining insights into the origins of POM in the ODZ. Since the within-ODZ Prochlorococcus assimilate nitrite, we used the resulting highly depleted d15N signal to trace organic matter of cyanobacterial origin to medium sized particles at the secondary chlorophyll maximum (SCM), and to >500 µm particles directly below the SCM. This organic nitrogen was consumed in the upper ODZ. Other POM maxima were seen at the zooplankton vertical migration maxima with the increase in POM marked in the 5-20 µm fraction. In the deep ODZ, below the zooplankton migration depth, POM concentrations in the 5-20 µm fraction were unusually small, the C:N ratios were extremely high (>20), and d15N was enriched (8-12‰), indicating degraded material. In deep samples, d13C was more depleted in larger particle size and correlated with enriched d15N, indicating increased degradation in 53-500 µm particles. This trend suggests an additional source of small particles, such as from in situ production, rather than just the fragmentation of large particles.