Dissolved oxygen is often considered the most important single chemical species in the ocean. Despite its central importance to understanding the biogeochemistry of the ocean, the accurate measurement of oxygen in the marine environment remains surprisingly challenging. Commercially available “optode” oxygen instruments are often plagued by data drift issues tied to material choices of the sensing membrane. This is especially true over long instrument deployment periods and in extreme environments. C-DEBI provided funds to build and deploy an optode-like instrument utilizing a UW-developed oxygen crystalline sensing material with superior long-term stability. To successfully meet the projects goals we built the VentO2 instrument, wrote control software for realtime acquisition and data processing, and deployed it on multiple dives using the ROV Jason II at Axial Seamount. We used a spectrometer-based measurement approach for the VentO2, which is a more challenging, but ultimately more powerful approach than the phase measurement used in optodes. The VentO2 instrument was successfully tested for functionality in the laboratory, and successfully recorded oxygen and temperature data on five separate dives to Axial Seamount on the Juan de Fuca Ridge. The resulting data was precise, but not accurate-- it tracked the trends in oxygen concentration as measured by the onboard reference instrument, but the magnitude of the data was offset. While we are encouraged by the success to date on a very limited budget, we acknowledge that much additional work is necessary to move the instrument beyond a working proof of concept.