Diffusion of oxygen (O) in amorphous silicon dioxide (SiO2) was investigated by means of Si3N4/natSinatO2/28Si18O2/28Si isotope heterostructures grown by thermal oxidation and plasma enhanced chemical vapour deposition. Diffusion experiments with and without a silicon nitride (Si3N4) cap, which serves as diffusion barrier for the gasses in the ambient, were performed. In particular, we determined the impact of the ambient gas, of the thickness of the isotopically enriched SiO2 layer, and of the annealing time and temperature on diffusion. Our results are compared with data given in the literature on oxygen and silicon diffusion in silica and are discussed in the framework of the experimental conditions established at the sample surface and at the buried 28Si18O2/28Si interface. Taking into account a point defect model that is predicted by recent atomistic simulations all experimental results can be explained consistently.