This study reports on the investigation and characterization of the different emitting centers within SiO2 codoped by Er3+ ions and silicon-excess. Erbium doped silicon-rich silicon oxide (SRSO:Er) thin films, fabricated by magnetron cosputtering at 500 °C, were analyzed by means of cathodoluminescence. The CL spectra of SRSO, Er-doped SiO2 and SRSO:Er were recorded and compared for various annealing temperatures. It was found that some specific optically-active point-defects called silicon-oxygen-deficient centers (SiODCs) are present in all kinds of samples. In the layers containing some excess Si, the phase separation between Si nanoclusters (Si-ncs) and SiO2 is observed when the annealing temperature reaches and exceeds 900 °C. The formation of Si-nc increases with annealing at the expense of SiODCs that was assumed to act as seeds for the growth of Si-nc. For SRSO:Er samples, the contribution of SiODCs overlaps that due to Er3+ transitions in the visible range. The emissions from SiODCs are drastically reduced when an SRSO sample is doped with Er ions, whereas the Er emissions in the visible range start to be distinctly observed. We propose a scenario of energy transfer from SiODCs toward the Er ions, especially as the emissions from the Si-based entities (SiODCs, Si-nc) and from some transitions of Er ions are located in a same visible broad range.