The photoluminescence (PL) properties of silanone, double bondSidouble bondO, and dioxasilyrane, double bondSi(O-2), in substoichiometric silicon oxide layers have been investigated by high-level ab initio calculations. The calculated 2.3 and 1.6 eV PL energies are evidenced to correspond to transitions from two lowest excited singlet states in silanone. Additional broadening of the PL bands is due to an involvement of the ground-state double bondSidouble bondO stretching and bending vibrations with frequencies of similar to1300 and similar to360 cm-1, respectively. As dioxasilyrane group is excited to the S-2 (2(1)A(1)) states, five excited states 1(1)B(2), 2(1)A(1), 1(1)A(2), 3(1)A(1), and 1(1)B(1) are involved in radiationless and radiative relaxation. The excitation initiates a rupture of the O-O bond and the deexcitation process drives through the activation of O-O stretching vibrations in the ground state (similar to630 cm-1). The radiative 3(1)A(1)-->1(1)A(1) transition contributes to PL in the range from 2.05 to 2.3 eV, while the 3(1)A(1)-->1(1)B(2) and 1(1)B(1)-->1(1)A(1) transitions can produce PL bands at 1.7-1.8 eV. The calculated results are compared with green PL bands experimentally observed for a wide range of nanoscale silicon and silicon oxide materials.