Luminescence from surface oxidized silicon nanocrystals (Si-nc) is an intriguing phenomenon with great potential for applications. Full consensus on the source of optical emission is not yet reached despite the huge research effort devoted to this topic. In this paper we report new evidence supporting a defect-related emission mechanism. In particular we propose a defect-based model to interpret the photoluminescence (PL) emission from partially oxidized Si-nc after thermal annealing in air. The PL emission is first studied in the time-dependent regime, using an approach based on discrete light-emitters. The results of this analysis are then exploited for elucidation of continuous wave (CW) data. Using our model the dependence of the PL emission spectrum on the excitation power and the shortening of the time decays at increasing pump power can be explained in terms of the different saturation behavior of luminescent defects. Moreover a rate equation approach for the coupled system composed by the Si-nc core and the associated luminescent defects is used to interpret the luminescence rise-time data under pulsed excitation. In this way the consistency of our picture is verified. Attribution of luminescent defects to non-bridging hole centers combined with oxygen vacancies in the oxide layer capping the Si-nc allows for a preliminary understanding of the increase of luminescence emission intensity and of the variation of its spectral features with the thermal annealing conditions.