Okmok volcano erupted on July 12, 2008, following an 11-year hiatus. Detailed inspection of the syn-eruptive seismograms revealed the presence of an ultra long-period mode at a frequency of 1.7 mHz, which is not a characteristic of the background seismic noise at Okmok. Data collected by the National Oceanic and Atmospheric Administration Geostationary Operational Environmental Satellite (GOES) and National Aeronautical and Space Administration Moderate Resolution Imaging Spectroradiometer (MODIS) sensors displayed the propagation of a vigorous ash-and-steam plume up to about 17 km above sea level. We suggest that the observed ultra long-period signals represent the response of the seismometer to changes in gravity associated with buoyancy oscillations set off in the lower atmosphere above Okmok by the emplacement of the eruption column. Calculations based on simple modeling of these effects allowed estimation of peak atmospheric pressure perturbations associated with the eruption of less than 1 mbar.