In 2004, Ohtomo and Hwang discovered that an electron gas is created at the interface between insulating and compounds. Here we show that the generation of a conducting electron gas is related to an orbital reconstruction occurring at the interface. Our results are based on extensive investigations of the electronic properties and of the orbital structure of the interface using x-ray absorption spectroscopy. In particular, we find that the degeneracy of the Ti states is fully removed and that the Ti levels become the first available states for conducting electrons.