  Recent experiments reported giant magnetoresistance at room temperature in
LaOMnAs. Here a density functional theory calculation is performed to
investigate magnetic properties of LaOMnAs. The ground state is found to be the
G-type antiferromagnetic order within the $ab$ plane but coupled
ferromagnetically between planes, in agreement with recent neutron
investigations. The electronic band structures suggest an insulating state
which is driven by the particular G-type magnetic order, while a metallic state
accompanies the ferromagnetic order. This relation between magnetism and
conductance may be helpful to qualitatively understand the giant
magnetoresistance effects.
