  The understanding of the Physics underlying the performances of organic
spin-valve devices is still incomplete. According to some recent models, spin
transport takes place in an impurity band inside the fundamental gap of organic
semiconductors. This seems to be confirmed by recent experiments performed with
La$_{0.7}$Sr$_{0.3}$MnO$_3$/Alq$_3$/AlO$_x$/Co devices. The reported results
suggest a possible correlation between the magnetoresistance and the variable
oxygen doping in the Alq$_3$ spacer. In this paper we investigate by means of
first-principles calculations the electronic and magnetic properties of O$_2$
molecules and ions in Alq$_3$ films to establish whether oxygen plays any
important role for spin transport in
La$_{0.7}$Sr$_{0.3}$MnO$_3$/Alq$_3$/AlO$_x$/Co devices. The conclusion is that
it does not. In fact, we show that O$_2$ molecules do not form an impurity band
and there is no magnetic interaction between them. In contrast, we suggest that
spin-transport may be enabled by the direct exchange coupling between Alq$_3^-$
ions.
