  The electronic structure and magnetism in the sodium nickelate NaNiO_2 in the
low-temperature phase is studied from density-functional calculations using the
linear muffin-tin orbitals method. An antiferromagnetic solution with a
magnetic moment of 0.7 m_B per Ni ion is found. A ferrodistorsive orbital
ordering is shown to occur due to the Jahn-Teller distortion around the Ni^{3+}
ion in agreement with the orbital ordering inferred from neutron diffraction.
While the intralayer exchange is ferromagnetic, the interlayer exchange is
weakly antiferromagnetic, mediated by a long Ni-O-Na-O-Ni superexchange path.
