The effects of substrate preparation on the structure and orientation of MgO films grown on (001) GaAs using pulsed laser deposition has been investigated. Textured MgO films displaying a (111)MgO‖(001)GaAs orientation relation with x‐ray rocking curve full width at half maximum (FWHM) values as low as 1.8° were obtained in cases where the native GaAs surface oxide was only partially desorbed prior to growth. Reflection high‐energy electron diffraction, transmission electron microscopy (TEM), and x‐ray pole figure analysis of these films reveals a preferential orientation within the plane of the substrate: [11¯0]MgO‖[11¯0]GaAs and [112¯]MgO‖GaAs. An interfacial layer (∼5 nm thick) was observed in high resolution TEM analysis, and was attributed to a remnant native GaAs oxide layer. Complete desorption of the native GaAs oxide at ∼600 °C in vacuum prior to MgO growth led to significant surface roughening due to Langmuir evaporation, and resulted in randomly oriented polycrystalline MgO films. Growth of MgO on Sb‐passivated GaAs substrates, which provided smooth, reconstructed surfaces when heated to 350 °C in vacuum, resulted in cube‐on‐cube oriented films [i.e., (001)MgO‖(001)GaAs,MgO‖GaAs] with x‐ray rocking curve FWHM values as low as 0.47°. TEM analysis of the cube‐on‐cube oriented films revealed evidence of localized strain fields at the MgO/GaAs interface, indicating the presence of misfit dislocations in the MgO layer.