  Thin films of transition metal oxides open up a gateway to nanoscale
electronic devices beyond silicon characterized by novel electronic
functionalities. While such films are commonly prepared in an oxygen
atmosphere, they are typically considered to be ideally terminated with the
stoichiometric composition. Using the prototypical correlated metal SrVO$_3$ as
an example, it is demonstrated that this idealized description overlooks an
essential ingredient: oxygen adsorbing at the surface apical sites. The oxygen
adatoms, which persist even in an ultrahigh vacuum environment, are shown to
severely affect the intrinsic electronic structure of a transition metal oxide
film. Their presence leads to the formation of an electronically dead surface
layer but also alters the band filling and the electron correlations in the
thin films. These findings highlight that it is important to take into account
surface apical oxygen or -- mutatis mutandis -- the specific oxygen
configuration imposed by a capping layer to predict the behavior of ultrathin
films of transition metal oxides near the single unit-cell limit.
