Power transformers are critical components of power system networks. Failure of this equipment risks the entire system, resulting in power outages and income losses. The reliability of this device is generally determined by its insulating quality. Hence, it is essential to check the transformer's insulation oil condition regularly. Traditional test techniques for assessing the condition of power transformers insulation oil have several drawbacks. Optical fiber-based sensors for monitoring transformer oil conditions have been designed to address such issues. This paper presents a brief review of optical fiber sensors for measuring oil level, oil aging assessment, moisture content and temperature in transformer oil. This review shall be of benefit to both engineers and researchers who are doing research in this field.