This work examines oxidative transformation of methane on 1.5 mol% Sr2+/La2O3 catalyst with nickel loadings up to 55 mol% nickel, prepared by wetness impregnation. The solids were characterized by X-ray diffraction, infrared spectroscopy, temperature-programmed reduction, BET surface area measurements, and thermogravimetric analysis. The NiO/Sr2+-La2O3 catalysts show a strong nickel oxide interaction with La2O3, involving anionic vacancies or structural defects that induce LaSrNiOx solid solutions with island-like structure formation. Two different reaction mechanisms are suggested depending on nickel loading. The 1.5 mol% Sr2+/La2O3 catalysts (0% Ni) favor oxidative coupling of methane (OCM) to C-2 hydrocarbons, whereas the Ni-modified ones favor syngas formation through partial oxidation of methane (POM). Anionic vacancies and coordinatively-unsaturated oxygen atoms (O-CUS) leading to hydroxycarbonate phase formation are proposed. Nickel carbide and hydroxycarbonate phases influence on syngas formation is analyzed.