Here we report that nanoclay-polyphenol-polymer hybrid sensing channels can greatly enhance the sensitivity for hazardous reactive oxygen species (ROS) in organic field-effect transistor (OFET) sensors. The hybrid layers were prepared by introducing nanoclay into the binary mixtures of poly(3-hexylthiophene) (P3HT) and rutin (a polyphenol) at various weight ratios. The presence of nanoclay improved the P3HT crystallinity in the hybrid films, which contributed to the increased drain current and well-maintained hole mobility even at the reduced amount of charge-transporting P3HT part. At the best composition (P3HT:rutin:nanocaly = 10:1:2 by weight), the OFETs with the hybrid layers were able to sense even 1 nM superoxide (a ROS) and exhibited pronouncedly enhanced sensitivity compared to those without nanoclay. The morphology investigation disclosed that rutin-nanoclay complexes formed in the hybrid films might be responsible for the enhanced sensitivity, because they let more rutin molecules protrude on the surface of channel layers for reactions with superoxide.