Optical O2 measurements with fiber-optic, planar or particle-based opt(r)odes have become widespread in aquatic science since the first O2 microopt(r)odes were introduced in 1995. A wide variety of measuring systems are now available both for lab-based and in situ field measurements at spatial scales ranging from µm to m’s, and temporal scales ranging from <1 s to several months. Furthermore, the market has seen an explosion in commercially available optical O2 sensor and measuring systems, which complement or have replaced electrochemical O2 sensors in several areas of application. In this chapter, we give a broad overview of measuring schemes and instruments used for optical O2 sensing, and illustrate their application in various aquatic environments. We also address demands and challenges related to optical O2 sensing in aquatic systems.