Deposition of two-dimensional (2D) materials onto catalyst surfaces is known to alter the adsorption energies of active sites due to the nanoconfinement effect. Traditionally, these 2D catalyst heterostructures were prepared by depositing a 2D material onto a pristine metallic surface. Preparing well-defined 2D monolayers, instead, on metal-oxide surfaces is challenging, although it is possible via O₂ intercalation by oxidizing a metal substrate underneath. Several studies demonstrate this intercalative behavior of 2D covers, however, without the preparation of ordered structures, which are imperative for defining fundamental reaction mechanisms in confined space. We report the successful preparation and characterization of a well-defined, ultrathin cuprous oxide-like film grown between h-BN and Cu(111). The confined surface oxide adopts a "Cu₂O-like" structure resembling the well-studied "44" Cu₂O structure, although the oxidation temperature is surprisingly lower than its uncovered oxide counterpart and the h-BN layer remains intact following oxidation. Our experimental results, backed by theoretical simulations, outline the development of a heterostructure with an h-BN/metal-oxide interface as a model system, utilizing a preparation method likely transferable to a wide range of 2D/metal heterostructures and opening the door to new catalyst designs.