Oxytocin receptor (OTR) plays critical roles in the development and expression of social behavior. Previous studies suggested that OTR expression is developmentally regulated with peak cortical expression in early postnatal period. However, quantitative understanding of OTR expression changes across different brain regions remains largely unknown. Thus, we examined the expression patterns of OTR positive cells throughout the whole brain at postnatal (P) periods (P7, 10, 14, 18, 21, 28) and in adulthood (P56) using transgenic reporter mice (OTR-eGFP). We used serial two-photon tomography to image the entire brain at cellular resolution and quantified fluorescently labeled cells with newly generated 3D postnatal brain templates at P7, 14, 21, and 28. We found significant heterogeneity in temporal pattern of OTR expression in brain regions including cortex. We then identified that transient OTR expression is mainly driven by OTR downregulation, not by cell death, using OTR-Cre:Ai14 for cumulative labeling. Lastly, we found significant delay of cortical OTR peak at P21 in OTR heterozygote mice (OTRvenus/+). We created a website to share the high-resolution imaging data as community resource for further data mining. In summary, our result provides essential quantitative data to understand postnatal OTR expression in the mouse brain.