  We investigated the influence of isotropic strain on the type of the magnetic
ground-state and the Fermi surface for the structurally-equivalent CeRuPO and
CeOsPO crystals with slightly different lattice parameters. The two systems
exhibit different magnetic orderings at zero strain. According to the phase
diagram of CeRuPO under pressure the difference might be due to the different
Ce-Ce inter-atomic distances. We applied {\it ab-initio} calculations based on
the density-functional theory and the generalized-gradient approximation with
an additional Coulomb repulsion (GGA+$U$), which indeed reveal a significant
impact of the strain and the effective $U$ parameter on the magnetic ground
state. However, it is demonstrated that the difference is more likely related
to the details in the Fermi surface.
