We examine spectral time series of the transition region line O v 629 \AA, observed with the Coronal Diagnostic Spectrometer (CDS) on the SoHO spacecraft in July 1997. Both Fourier and wavelet transforms have been applied independently to the analysis of plume oscillations in order to find the most reliable periods. The wavelet analysis allows us to derive the duration as well as the periods of the oscillations. Our observations indicate the presence of compressional waves with periods of 10-25 minutes. We have also detected a 11 \pm 1 minute periodicity in the network regions of the north polar coronal hole. The waves are produced in short bursts with coherence times of about 30 minutes. We interpret these oscillations as outward propagating slow magneto-acoustic waves, which may contribute significantly to the heating of the lower corona by compressive dissipation and which may also provide enough energy flux for the acceleration of the fast solar wind. The data supports the idea that the same driver is responsible for the network and plume oscillations with the network providing the magnetic channel through which the waves propagate upwards from the lower atmosphere to the plumes.