  We report on a detailed investigation of the itinerant ferromagnets LaCoAsO,
PrCoAsO and SmCoAsO performed by means of muon spin spectroscopy upon the
application of external hydrostatic pressures $p$ up to $2.4$ GPa. These
materials are shown to be magnetically hard in view of the weak dependence of
both critical temperatures $T_{C}$ and internal fields at the muon site on $p$.
In the cases $R$ = La and Sm, the behaviour of the internal field is
substantially unaltered up to $p = 2.4$ GPa. A much richer phenomenology is
detected in PrCoAsO instead, possibly associated with a strong $p$ dependence
of the statistical population of the two different crystallographic sites for
the muon. Surprisingly, results are notably different from what is observed in
the case of the isostructural compounds $R$CoPO, where the full As/P
substitution is already inducing a strong chemical pressure within the lattice
but $p$ is still very effective in further affecting the magnetic properties.
