X-ray Emission from O stars Young O stars are strong, hard, and variable X-ray sources, which strongly affects their circumstellar and galactic environments. After < 1 Myr, these stars settle down to become steady sources of soft X-rays. I will use high-resolution X-ray spectroscopy and MHD modeling to show that young O stars like theta-1 Ori C are well explained by the magnetically channeled wind shock scenario. After their magnetic fields dissipate, older O stars produce X-rays via shock-heating in their unstable stellar winds. Here too I will use X-ray spectroscopy and numerical modeling to confirm this scenario. In addition to elucidating the cause and nature of O star X-ray emission, modeling of the high-resolution X-ray spectra of O supergiants provides strong evidence that mass-loss rates in these stars have been overestimated.