In this study, the authors investigate the impact of radical oxygen plasma on nitrided and annealed atomic layer deposited (ALD) SiO2 as a thick gate oxide (1.65-3 V) with a high-k/metal gate transistor. Time-dependent-dielectric-breakdown voltage, secondary ion mass spectroscopy (SIMS), and x-ray photoelectron spectroscopy (XPS) studies were conducted, and the results are discussed for nitrided and annealed ALD SiO2 with and without radical oxygen plasma exposure. Atomistic material simulations were performed to understand the reactions between oxygen radicals and silicon oxynitride (SiON). Our key findings from SIMS and XPS show that radical oxygen plasma exposure led to a 34% nitrogen loss from the thick SiON gate oxide and damaged the gate oxide, resulting in a 1 V reduction in the dielectric breakdown voltage. Atomistic material simulation results also show that atomic oxygen can react with Si-(ON)-Si to form Si-O-Si bonds and mobilize NO into the interstitial space. Similarly, when O atoms and O2 molecules are placed near N clusters, spontaneous diffusion of N2 into the interstitial space occurs. Diffusion barrier calculations further indicate a barrier energy of less than 0.5 eV in SiO2 and SiON, which can lead to the out-diffusion of NO and N2 from SiO2 and SiON. These compositional changes may result in increased leakage and a degraded breakdown voltage. The increased gate leakage and degraded breakdown voltage is attributed to the compositional changes in SiON.