There is growing interest in understanding the properties of SiC-SiO2 interfaces, which can be formed by oxidation of silicon carbide surfaces. Here, we used variable cell shape ab initio molecular dynamics to investigate the structural and electronic properties of crysta phases of silicon oxycarbide which could appear within such interfaces. We find that car oxygen single bonds may remain stable inside a silicon oxide matrix. For the Si2CO6 compo there are at least two crystalline phases, both having large bulk modulii and wide bandgaps.