GaAs is grown by molecular beam epitaxy (MBE) on composite GaAs-SiO2 nucleation layers fabricated on Si substrates. The composite surface (CS) is formed by an initial deposition of GaAs islands by MBE, followed by oxidation of the bare Si regions surrounding the islands. The oxidation is performed at 265°C in the afterglow of a microwave induced plasma. The fabrication of the composite GaAs-SiO2 nucleation layer is compatible with the epitaxy process. GaAs growth on CS's is performed to study the seeded lateral overgrowth of GaAs on the SiO2. We find single crystal GaAs on SiO2 regions up to 50 nm wide. On the SiO2 a large density of twins is present. The GaAs seeds are not pseudomorphic and misfit dislocation segments constitute a source of threading defects. The potential of the technique for GaAs-on-Si improvement is discussed.