We describe a method for determining net annual biological oxygen production in the euphotic zone of the ocean using remote in situ measurements of oxygen and nitrogen gas. Temperature, salinity, oxygen, and total dissolved gas pressure were measured every 2 hours at 10-m depth on a mooring at the Hawaii Ocean time series during the year 2005. Since dissolved N2 is effectively inert to biological processes it can be used as a tracer for the physical mechanisms affecting the O2 concentration in an upper ocean model of gas concentrations. We determine a net biological oxygen production in the surface mixed layer of 4.8 ± 2.7 mol m−2 yr−1. The most important term in the mixed-layer mass balance other than biological oxygen production is the flux of oxygen across the air–water interface to the atmosphere. Simultaneous glider surveys of the O2 field measured in a companion paper (Nicholson et al., 2008) yield net biological oxygen production below the mixed layer of 0.9 ± 0.1 mol O2 m−2 yr−1. The upper-ocean mass balance also includes a potential contribution from diapycnal mixing of O2 into the pycnocline of 0–0.8 mol O2 m−2 yr−1. Assuming that the net biological oxygen production over a period of a year or longer is stoichiometrically related to net biological carbon production and export via ΔO2/ΔC = 1.45, the biological carbon flux from the euphotic zone at HOT is 4.1 ± 1.9 mol C m−2 yr−1 in 2005, with roughly 80% of the carbon production originating in the mixed layer. Annual estimates of this flux (the ocean's “biological carbon pump”) have been determined experimentally in only a few locations of the ocean because of the labor and expense involved in repeated ship board measurements. With this new in situ method, it may now be possible to better quantify the global distribution of the net annual biological carbon export, a prominent mechanism of carbon cycle feedback in response to climate change, both in the past and future.