Detailed mechanisms that control the formation of output from copper vapor laser (CVL) oscillators are investigated. Measurements of the spatial and temporal evolution of gain in a CVL amplifier and a CVL oscillator show that a short period of high gain that occurs at the beginning of the inversion period is the dominant feature. This leads to the formation of a burst of amplified spontaneous emission (ASE), whose subsequent propagation and amplification leads to all observable CVL output. The spatial characteristics of this initial burst of ASE are shown to be strongly dependent on the operating conditions of the laser. The implications of this description of CVL output for the design of unstable resonators and oscillator–amplifier systems is discussed.