Condition monitoring of power transformers is of great importance for the timely detection of incipient faults to avoid potential malfunctioning. Transformer insulating oil contains about 70% of diagnostic information, and a dramatic rise in oil temperature may drastically reduce the lifetime of power transformers, and thus the temperature of the oil is considered the most crucial parameter that has to be monitored continuously in real-time. Compared to traditional temperature measurement methods used in transformer condition monitoring, distributed optical fiber sensors have inherent advantages of immunity to electromagnetic interference and insulation at high-voltage levels, and they offer spatially resolved temperature monitoring with high accuracy and sensitivity. In this study, optical fiber-based distributed temperature measurement of a fully energized 100 kVA distribution transformer is demonstrated by using two different techniques: Optical Frequency Domain Reflectometry (OFDR) and Fiber Bragg Grating (FBG) sensor array. The fiber sensors are robust for a safe long-term installation into oil-filled distribution transformers during manufacturing, and they can withstand heat runs, long-term hot oil immersion, and transformer vibration. The internal transformer temperature is monitored during standard thermal tests prior to installation on the distribution system. The test results show very good agreement between the standard thermocouple and proposed distributed fiber temperature sensors, providing transformer manufacturers with new insights into the distribution of temperatures internal to their commercial products.