The several processes required to achieve Er luminescence in Si are investigated. In particular, the role of Er-O interactions to obtain the incorporation of high Er concentrations, electrically and optically active, in crystalline Si is addressed. Multiple Er and O implants were performed on n-type (100) Si crystals to obtain flat concentrations of ∼1×1019 Er/cm3 and ∼1×1020 O/cm3 over an ∼2-μm-thick layer. These implants produced also a 2.3-μm-thick amorphous Si (a-Si) layer. A subsequent thermal treatment at 620 °C for 3 h induced the epitaxial regrowth of the whole layer and the incorporation of both Er and O in a good-quality single crystal. A further annealing at 900 °C for 30 sec produced the electrical activation of the implanted Er in the presence of O, with an Er donor concentration of ∼8×1018/cm3 over an ∼1.8-μm-thick layer. This value is more than two orders of magnitude above the maximum Er donor concentration reported in the literature, demonstrating the crucial role of O in increasing the electrically active Er concentration in crystalline Si. The optical efficiency of this sample has been studied by photoluminescence. It is seen that an enhancement by a factor of ∼6 with respect to the literature data is obtained. Moreover, studies on the photoluminescence intensity as a function of the pump power give important information on the mechanisms underlying Er luminescence in Si and its competing phenomena. These data are presented and discussed. A plausible model based on the previous results is also presented.