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The conductance confined at the interface of complex oxide heterostructures provides new opportunities to explore nanoelectronic as well as nanoionic devices. Herein we show that metallic interfaces can be realized in SrTiO₃-based heterostructures with various insulating overlayers of amorphous LaAlO₃, SrTiO₃, and yttria-stabilized zirconia films. On the other hand, samples of amorphous La_7/8Sr_1/8MnO₃ films on SrTiO₃ substrates remain insulating. The interfacial conductivity results from the formation of oxygen vacancies near the interface, suggesting that the redox reactions on the surface of SrTiO₃ substrates play an important role.