Graphene/metal heterojunctions are ubiquitous in graphene-based devices and, therefore, have attracted increasing interest of researchers. Indeed, the literature on the field reports apparently contradictory results about the effect of a metal on graphene doping. Here, we elucidate the effect of metal nanostructuring and oxidation on the metal work function (WF) and, consequently, on the charge transfer and doping of graphene/metal hybrids. We show that nanostructuring and oxidation of metals provide a valid support to frame WF and doping variation in metal/graphene hybrids. Chemical vapour-deposited monolayer graphene has been transferred onto a variety of metal surfaces, including d-metals, such as Ag, Au, and Cu, and sp-metals, such as Al and Ga, configured as thin films or nanoparticle (NP) ensembles of various average sizes. The metal-induced charge transfer and the doping of graphene have been investigated using Kelvin probe force microscopy (KPFM), and corroborated by Raman spectroscopy and plasmonic ellipsometric spectroscopy. We show that when the appropriate WF of the metal is considered, without any assumption, taking into account WF variations by nanostructure and/or oxidation, a linear relationship between the metal WF and the doping of graphene is found. Specifically, for all metals, nanostructuring lowers the metal WF. In addition, using gold as an example, a critical metal nanoparticle size is found at which the direction of charge transfer, and consequently graphene doping, is inverted.