The charge compensation effects induced by aliovalent doping were studied in the monoclinic CdSiO3. An interstitial oxide ion may be feasible in this host to provide the extra negative charge required by the R3+ doping. The oxide was studied in CdSiO3 using density functional theory (DFT) calculations and synchrotron radiation (SR) luminescence spectroscopy. The crystal structure of this host was significantly modified by the interstitial oxide. The experimental band gap energy (Eg) was perfectly reproduced by the calculations and intrinsic electron traps were revealed. Defect levels were found also in the interstitial oxide containing host, however their role has to be studied further.