  Using state-of-the-art first-principles calculations we study the magnetic
behaviour of CeOFeAs. We find the Ce layer moments oriented perpendicular to
those of the Fe layers. An analysis of incommensurate magnetic structures
reveals that the Ce-Ce magnetic coupling is rather weak with, however, a strong
Fe-Ce coupling. Comparison of the origin of the tetragonal to orthorhombic
structural distortion in CeOFeAs and LaOFeAs show marked differences; in
CeOFeAs the distortion is stabilized by a lowering of spectral weight at the
Fermi level, while in LaOFeAs by a reduction in magnetic frustration. Finally,
we investigate the impact of electron doping upon CeOFeAs and show that while
the ground state Fe moment remains largely unchanged by doping, the stability
of magnetic order goes to zero at a doping that corresponds well to the
vanishing of the N\'eel temperature.
