Una K. Miller ^1* Kristen Fogaren ² Dariia Atamanchuk ³ Clare Johnson ⁴ Jannes Koelling ^5,6 Isabela Le Bras ⁷ Margaret Lindeman ⁸ Hiroki Nagao ⁷ David P. Nicholson ⁷ Hilary Palevsky ² Ellen Park ⁷ Meg Yoder ² Jaime B. Palter ¹

• ¹ University of Rhode Island, Kingston, United States
• ² Boston College, Chestnut Hill, Massachusetts, United States
• ³ Dalhousie University, Halifax, Nova Scotia, Canada
• ⁴ Scottish Association For Marine Science, Oban, United Kingdom
• ⁵ University of Washington, Seattle, Washington, United States
• ⁶ Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration (NOAA), Seattle, Washington, United States
• ⁷ Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, United States
• ⁸ University of Southampton, Southampton, Hampshire, United Kingdom

Increasing interest in the deployment of optical oxygen sensors, or optodes, on oceanographic moorings reflects the value of dissolved oxygen (DO) measurements in studies of physical and biogeochemical processes. Optodes are well-suited for moored applications but require careful, multistep calibrations in the field to ensure data accuracy. Without a standardized set of protocols, this can be an obstacle for science teams lacking expertise in optode data processing and calibration.Here, we provide a set of recommendations for the deployment and in-situ calibration of data from moored optodes, developed from our experience working with a set of 60 optodes deployed as part of the Gases in the Overturning and Horizontal circulation of the Subpolar North Atlantic Program (GOHSNAP). In particular, we detail the correction of drift in moored optodes, which occurs in two forms: (i) an irreversible, time-dependent drift that occurs during both optode storage and deployment and (ii) a reversible and pressure-and-time-dependent drift that is detectable in some optodes deployed at depths greater than 1000 m. The latter is virtually unidentified in the literature yet appears to effect a low-bias in measured DO on the order of 1 to 3 µmol kg -1 per 1000 m depth, appearing as an exponential decay over the first days to months of deployment. Comparisons of our calibrated DO time series against serendipitous mid-deployment conductivity-temperature-depth (CTD)-DO profiles, as well as biogeochemical (BGC)-ARGO float profiles, suggest the protocols described here yield an accuracy in optode-DO of ∼1%, or about 2.5 to 3 µmol kg -1 . We intend this paper to serve as both documentation of the current best practices in the deployment of moored 1 optodes as well as a guide for science teams seeking to collect high-quality moored oxygen data, regardless of expertise.