We report a detailed investigation of RCoPO (R = La, Pr) and LaCoAsO
materials performed by means of muon spin spectroscopy. Zero-field
measurements show that the electrons localized on the Pr3+ ions do not
play any role in the static magnetic properties of the
compounds. Magnetism at the local level is indeed fully dominated by
the weakly itinerant ferromagnetism from the Co sublattice only. The
increase of the chemical pressure triggered by the different ionic
radii of La3+ and Pr3+, on the other hand, plays a crucial role in
enhancing the value of the magnetic critical temperature and can be
mimicked by the application of external hydrostatic pressure up to 24
kbar. A sharp discontinuity in the local magnetic field at the muon
site in LaCoPO at around 5 kbar suggests a sizable modification in the
band structure of the material upon increasing pressure. This scenario
is qualitatively supported by ab initio density-functional-theory
calculations.