A single-mode fiber (SMF) sensor for detecting coconut oil adulteration is proposed. Coconut oil is commonly used in cooking but health problems are caused by its adulteration. The lateral offset approach to the SMF-SMF displacement sensor was employed in this experiment to analyze the sensing responses of adulterant concentrations in coconut oil. The offset distances of the sensing probe were set at 6.47 μm, 11.57 μm, and 14.64 μm. Pure coconut oil, paraffin oil, and palm oil have an initial refractive index of 1.4481, 1.4585, and 1.4634, respectively. Upon completion, the highest sensitivity was observed at a lateral offset distance of 14.64 μm. These values were 0.286 dBm/mol for palm oil detection and 0.045 dBm/mol for paraffin oil detection. The findings of these experiments also showed that the larger the offset distance, the greater the sensitivity of the fiber sensor.