  By means of a Wannier projection within the framework of density functional
theory, we are able to identify the modified c-axis hopping and the energy
mismatch between the cation bands as the main source of the $t_{2g}$ splitting
around the $\Gamma$ point for oxide heterostructures, excluding previously
proposed mechanisms such as Jahn-Teller distortions or electric field
asymmetries. Interfacing LaAlO$_3$, LaVO$_3$, SrVO$_3$ and SrNbO$_3$ with
SrTiO$_3$ we show how to tune this orbital splitting, designing
heterostructures with more $d_{xy}$ electrons at the interface. Such an
"orbital engineering" is the key for controlling the physical properties at the
interface of oxide heterostructures.
