Sorption of oxygen in the pores of zeolite HY and 1H MAS NMR has been used to determine which Brønsted acid sites are accessible to oxygen. Large increases in the 1H spinning-sideband manifolds are observed at low temperatures for the supercage protons that can directly interact with the oxygen molecules; a much smaller increase in sideband intensity is seen for the sodalite protons. Dramatic reductions of the T1's of all the protons, of approximately 2 orders of magnitude, are observed at -150°C, in comparison to T1 measurements made in air at room temperature, or at -150°C for samples with adsorbed N2. The T1's of the supercage protons are, however, shorter than those measured for the sodalite protons. Both the decrease in the T1's, and the large sideband manifolds, are due to the dipolar coupling interactions with the unpaired electrons present on the O2 molecules. Second moment (M2) analysis is used to quantify the increase in the width of the 1H spinning sideband manifolds, in samples of zeolites HY and HZSM-5. Comparison between samples run in air and with fixed loading levels of oxygen allows changes in loading level with temperature to be detected and oxygen loading levels to be estimated. The contribution to M2 of the protons due to bulk magnetic susceptibility effects and from oxygen molecules that do not directly interact with the protons was estimated from the values of M2 extracted from the 29Si MAS NMR of the same samples. The contribution to M2 from the direct interaction of the protons with the oxygen molecules could then be estimated.