Efficient electrical spin injection into GaAs(110) semiconductor structures requires　ferromagnetic electrodes with perpendicular magnetization, as electron spins oriented normal to (110)-oriented quantum wells exhibit exceptionally long relaxation times. While MgO/CoFeB heterostructures provide highly spin-polarized tunneling currents and interfacial perpendicular magnetic anisotropy (PMA) on GaAs(001), implementing such spin injectors on GaAs(110) has remained unexplored. This because the reduced symmetry of the (110) surface is expected to hinder MgO(001) growth, whereas introducing thick amorphous oxide layers to decouple symmetry is known to severely degrade spin injection efficiency and increase junction resistance. Here we address this materials challenge by identifying the minimal interfacial conditions under which PMA emerges on GaAs(110). We find that retaining an ultrathin residual oxide correlates with the appearance of PMA after annealing at 300 °C, whereas PMA is absent when the oxide is removed or when the thermal budget is nonoptimal. Structural analysis indicates that the MgO layer exhibits local crystalline ordering, although its precise orientation cannot be uniquely determined from the present data. These results demonstrate that PMA on GaAs(110) is stabilized only within a narrow interfacial and thermal tolerance window.