xytocin (OT), a hypothalamic neuropeptide that acts as a neuromodulator in the brain, orchestrates a variety of animal behaviors. However, the relationship between brain OT dynamics and complex animal behaviors remains largely elusive, partly because of the lack of a suitable technique for its real-time recording in vivo. In this study, we developed an ultrasensitive fluorescent OT sensor named MTRIA_OT, which is composed of a medaka OT receptor (OTR) and a circularly permutated green FP (cpGFP)-based fluorescent module named MTRIA (Modular fluorescence unit fused with TRansmembrane region-to-IntrAcellular loop linkers). We demonstrate that MTRIA_OT-mediated in vivo fluorescence recording can report artificially-evoked OT responses as well as endogenously-controlled OT signals with a fast temporal resolution. Importantly, our analysis revealed that the temporal profiles of OT signals are highly variable depending on the behavioral context of the animal, and that the dynamics can be altered by perturbations, such as administration of anesthetic drugs, food deprivation, and aging. We also demonstrate the utility of MTRIA, the fluorescent module of MTRIA_OT, for the efficient development of a variety of GPCR-based fluorescent sensors. Together, our findings indicate that MTRIA_OT offers opportunities for the real-time detection of OT dynamics in the living brain, and potentially expands the repertoire of GPCR-based fluorescent sensors for extracellular ligands.