This work presents the fabrication and characterization of a fiber optic gas sensor utilizing Lossy Mode Resonances (LMR) achieved with a nanoscale polyphenylene oxide (PPO) coating. The PPO coating is applied to the cladding removed region of a multi-mode silica fiber, simultaneously serving as both the LMR support layer and the sensing material. The PPOcoated LMR sensor is exposed to various concentrations of ammonia as a case study. The sensor demonstrates a high sensitivity of 0.25 nm/ppm within the ammonia concentration range of 2.5 to 37.5 ppm. These results highlight the potential of this PPO-based fiber optic sensor for efficient and sensitive detection of hazardous compounds in different settings, ranging from medical to industrial.