  Comprehensive magnetic-field-orientation dependent studies of the
susceptibility and de Haas-van Alphen effect have been carried out on single
crystals of the filled skutterudites PrOs$_4$As$_{12}$ and LaOs$_4$As$_{12}$
using magnetic fields of up to 40~T. Several peaks are observed in the
low-field susceptibility of PrOs$_4$As$_{12}$, corresponding to cascades of
metamagnetic transitions separating the low-field antiferromagnetic and
high-field paramagnetic metal (PMM) phases. The de Haas-van Alphen experiments
show that the Fermi-surface topologies of PrOs$_4$As$_{12}$ in its PMM phase
and LaOs$_4$As$_{12}$ are very similar. In addition, they are in reasonable
agreement with the predictions of bandstructure calculations for
LaOs$_4$As$_{12}$ on the PrOs$_4$As$_{12}$ lattice. Both observations suggest
that the Pr 4$f$ electrons contribute little to the number of itinerant
quasiparticles in the PMM phase. However, whilst the properties of
LaOs$_4$As$_{12}$ suggest a conventional nonmagnetic Fermi liquid, the effects
of direct exchange and electron correlations are detected in the PMM phase of
PrOs$_4$As$_{12}$. For example, the quasiparticle effective masses in
PrOs$_4$As$_{12}$ are found to decrease with increasing field, probably
reflecting the gradual suppression of magnetic fluctuations associated with
proximity to the low-temperature, low-field antiferromagnetic state.
