Radiation dosimeters based on in situ oxygen plasma post-treated diamond thin films were fabricated in a simple sandwich configuration. Effects of deposition process, methane concentration, and in situ oxygen plasma post-treatment on the sensitivity of the devices to X-ray irradiation were investigated. X-ray response demonstrates that the cyclic deposition process could improve response sensitivity. The increase in methane concentration in the deposition gas mixture will worsen the irradiation response of the devices mainly resulted from the change of the orientation and purity of the films. X-ray photoelectron spectroscopy, photoluminescence, and Raman measurements suggest that in situ oxygen plasma post-treatment can effectively etch non-diamond phases and passivate the silicon-vacancy and nitrogen-vacancy defects of the diamond films, resulting in an increase in the sensitivity of the device by a factor of about 2. Time-dependent response to X-ray indicates that the extended period to achieve photocurrent signals stability for the devices is a limitation for promising applications in radiation dosimetry.