Using a low‐pressure reactive ion etching reactor in a pulsed radio‐frequency (rf) power mode the rise time of the plasma induced emission of O at 777 and 844 nm was monitored on a ms time scale. With a two‐photon excitation scheme the rise time of the oxygen atom ground state concentration was followed by measuring the laser induced fluorescence (LIF) at 844 nm. An absolute density measurement was performed by comparing the LIF signal in the reactor to the LIF signal of a known concentration of O in a microwave discharge. Using these time resolved measurements the reliability of optical emission actinometry in low pressure O2rf plasmas for a determination of relative atomic oxygen densities was examined. By comparing the rise times with the ignition time of the plasma it is concluded that both emission lines are caused by dissociative excitation. The consequences for the applicability of actinometry will be discussed. A comparison of experimental emission intensities to calculated ones suggests that highly energetic (e.g., secondary) electrons are responsible for the dissociative excitation of O2.