  We investigated \LAO\ - \STO\ heterointerfaces grown either in oxygen rich or
poor atmosphere by soft x-ray spectroscopy. Resonant photoemission across the
Ti L$_{2,3}$ absorption edge of the valence band and Ti 2p core level
spectroscopy directly monitor the impact of oxygen treatment upon the
electronic structure. Two types of Ti$^{3+}$ related charge carriers are
identified. One is located at the Fermi energy and related to the filling of
the \STO\ conduction band. It appears for low oxygen pressure only. The other
one is centered at E$_{B}$ $\approx$ 1 eV and independent of the oxygen
pressure during growth. It is probably due to defects. The magnitude of both
excitations is comparable. It is shown that low oxygen pressure is detrimental
for the Ti - O bonding. Our results shed light on the nature of the charge
carriers in the vicinity of the \LAO\ - \STO\ interface.
