The adsorption of oxygen on clean nickel (110) surfaces has been studied by low‐energy electron diffraction. Several successive structures are formed before the first monolayer is completed. These structures consist of both oxygen and nickel atoms, with each having more oxygen atoms and fewer nickel than the preceding. With increasing oxygen content the sticking probability becomes progressively lower, from about unity for the (1×2) structure to 0.01 for the final most oxygen‐rich structure. The temperature necessary for the degradation of any structure to that previously formed decreases from a value near the melting point of nickel for the (1×2) structure to about 250°C for the most oxygen‐rich structure. Normal cubic nickel oxide can be grown on top of the most stable of these oxygen‐nickel structures, but only at rather high temperatures and pressures. The oxide is oriented with a [100] direction normal to the (110) surface of the crystal, and with a [110] direction parallel to the [100] nickel direction in the surface plane.