Extra virgin olive oil is a product in great demand, due to its popularity as the healthiest cooking oil. Thanks to its popularity and high price, extra virgin olive oil is also a common target of adulterators who add other vegetable oils in order to increase the volume of the final product. This practice is specially harmful to consumers who choose olive oil due to dietary restrictions, in addition to directly affecting producers, due to unfair competition. Based on this scenario, this work presents a D-shaped optical fiber-based refractive index sensor to detect adulteration of olive oil caused by the addition of canola, corn, cotton, soybean and sunflower oils. The device fabrication and characterization processes are presented, detailing the instrumentation used and the methodology adopted during the measurements. The sensor sensitivity, measured against the added percentage of contaminants, was determined as 0.037 dB/%. The results show that the sensor is capable of detecting adulterations from additions of any combination of the five adulterants greater than 9% of the total volume.