Fast neuronal network oscillations in the gamma range (~30–80 Hz) have been implicated in higher brain functions such as memory formation and sensory processing. In hippocampal slice preparations gamma oscillations can be evoked by acetylcholine, which mimics cholinergic input from the septum. In this study, the O2 and energy demands of gamma oscillations were determined in acute mouse slices and in rat organotypic slice cultures by applying electrophysiology, O2 sensor microelectrodes and fluorescence imaging (NAD(P)H, FAD). It was shown that gamma oscillations decrease significantly at pO2 levels that do not affect neuronal population responses as elicited by electrical stimuli, which leads to the suggestion that they are highly O2 dependent. This is supported by the finding that gamma oscillations are associated with high levels of O2 consumption. Moreover, they require utilization of mitochondrial oxidative capacity near limit and are very sensitive to selective complex I inhibition by rotenone. The study highlights the importance of a functional understanding of mitochondria and their implications on activities of individual neurons and neuronal networks.