We have studied parylene-N and parylene-C for their use as substrates and gate dielectrics in OTFTs. Parylene-N films with a thickness of 300 nm show the required dielectric properties, as verified by breakthrough-voltage measurements. The surface roughness measured for 300 nm thick parylene-N films is 4–5 nm. However, initial growth of parylene depends on the subjacent surface. This results in different thicknesses on Au electrodes and substrate materials for thin films. Capping of micro-patterned Au-electrodes with a thin Al layer via lift-off results in homogenous parylene film thickness on the whole sample surface. OTFTs are fabricated on glass with parylene-N as a gate dielectric and pentacene as a semiconductor. The electrodes are patterned by photolithography enabling micrometer sized features. The contact resistance is extracted by variation of the channel length. Modification of the parylene dielectric layer surface by plasma treatment with oxygen after deposition allows shifting of the threshold voltage to more positive values, however at the cost of increasing hysteresis. OTFTs fabricated on thin parylene-C films can be peeled off and could result in flexible devices employing parylene-C foil as a substrate. For a foil thickness of 3–4 μm, operational devices can be bent down to radii less than 1 mm, e.g. in the range of cannulas. Operation of such OTFTs with parylene-C as a gate dielectric in liquids is demonstrated. The OTFT current can be modulated by the potential in the electrolyte as well as by the bottom gate potential. This allows for application of such OTFTs as sensors in medical devices.