Airglow oscillations (mostly 5577-Ā oxygen emission) which show a short-period (4--10 min) oscillation superimposed on a long-period oscillation (several hours) have frequently been observed. Some data show that the short-period oscillation is 'excited' by the long-period oscillation as the latter increases in amplitude. The effects often occur in the absence of impulsive energy sources. We propose a model in which the short-period oscillation is relatively localized and is associated with the natural (Brunt-Vaisala) oscillation of the atmosphere. This oscillation is triggered off by a long-period gravity wave through parametric excitation and nonlinear coupling.