We have developed a means to perform "on the fly" electron spin resonance (ESR) measurements of NBTI defect generation. The approach permits ESR measurements to be performed during NBTI stress void of any recovery contamination. We demonstrate that elevated temperature (100°C) and modest negative polarity oxide electric field (<5MV/cm) generates ESR spectra of E' oxide defects. (These defects are holes trapped in oxygen vacancies.) When similar measurements are made at elevated temperature and no oxide bias, E' center spectra are not observed. When ESR measurements are made with identical negative oxide bias at room temperature, E' center spectra are not observed. Furthermore, we demonstrate that the NBTI induced E' center spectrum disappears, a recovery phenomena, when the NBTI stressing condition is removed. These observations indicate that NBTI is triggered by inversion layer hole capture at an E' precursor site (an oxygen vacancy) which then leads to the depassivation of nearby interface states (Pb centers).