Electron spin resonance is used to study oxide damage centers generated by ionizing radiation in MOS capacitors as a function of bias and temperature during irradiation. Two centers are observed, an E' and an oxygen hole center. The concentration of E' centers is about three times as high for samples irradiated under positive gate bias as for those irradiated under negative bias. The concentration of E' centers is independent of sample temperature during irradiation indicating that some charge carriers created by irradiation are still mobile at low temperatures. The oxygen hole center density is not a function of bias indicating that they are relatively uniformly distributed throughout the bulk of oxide.