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Molecular beam epitaxy Ga-assisted synthesis of GaAs nanowires is demonstrated. The nucleation and growth are seen to be related to the presence of a Si O 2 layer previously deposited on the GaAs wafer. The interaction of the reactive gallium with the Si O 2 pinholes induces the formation of nanocraters, found to be the key for the nucleation of the nanowires. With Si O 2 thicknesses up to 30 nm⁠, nanocraters reach the underlying substrate, resulting into a preferential growth orientation of the nanowires. Possibly related to the formation of nanocraters, we observe an incubation period of 258 s before the nanowires growth is initiated.