This paper presents an analysis of the thermal behavior of a 15-kVA ONAN distribution transformer. Design, performance, and service-life improvements come into view from this study. The mathematical model, conceived as a 3-D solid-oil-environment extended system, considers all modes of heat transfer, including radiation. We computed oil velocity profile and oil and winding temperature distribution, patterns thereafter experimentally validated in accordance with the IEEE-1995 Loading Guide. Since this paper identifies hot-spot and flow patterns more accurately than its predecessors, and reveals a serious malfunction in refrigeration channels, it creates interesting possibilities for new designs in transformers below 75 kVA.