We have used a DFT + U approach to study the characteristics of a series of 3d transition-metal oxide monolayers (MLs) of M 2 O 3 stoichiometry (M = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) in freestanding and Au(111)-supported honeycomb structures. They include), and purely charge-transfer (Sc 2 O 3) oxides. We have found that besides a structural polarization, the interaction with the Au substrate induces an interfacial electron transfer and considerably modifies the electronic characteristics of the monolayers. In particular, in Ti 2 O 3 , V 2 O 3 , and Ni 2 O 3 , due to the purely d character of the relevant gap edges, these modifications may be rationalized in terms of a change of the cation oxidation state with respect to the corresponding bulk and freestanding ML reference. We propose a general conceptual framework that links the properties of the separate systems (band offsets and character of the states at the gap edges), the value of the interfacial electron transfer, and the electronic and energetic characteristics of the supported MLs. Our results and analysis furnish guidelines toward the tuning of electronic and magnetic characteristics beyond those imposed by the oxide stoichiometry, of direct interest for modern technologies.