In order to analyze neuronal computations in the first olfactory processing center, the olfactory bulb, we measured odor-evoked activity patterns across large numbers of neurons within the intact olfactory bulb of zebrafish using optical and electrophysiological methods. We found that the olfactory bulb performs multiple computations including a decorrelation of overlapping inputs, a multiplexing of complementary information, and gain control. Patterns of olfactory bulb output activity are reorganized during the initial phase of an odor response, resulting in a partial loss of the topographic representation of molecular features and in the decorrelation of activity patterns evoked by similar stimuli. Physiological, pharmacological, and computational results provide initial insights into the mechanisms underlying these computations.