  We present results of GGA+U calculations for the "d^0 magnet" RbO_2, where
magnetic properties are due to partially filled oxygen p orbitals. We show that
on-site interactions on the oxygen sites lead to a strong tendency towards the
formation of an orbitally polarized insulating state, in contrast to the
half-metallic behavior predicted for this class of compounds within pure
LDA/GGA. The obtained energy differences between different orbitally ordered
configurations are sizeable, indicating an orbital ordering temperature higher
than the antiferromagnetic Neel temperature of ~15 K. Our results demonstrate
the importance of correlation effects in p electron magnets such as RbO_2.
