  Filled skutterudite compounds have gained attention recently as an innovative
platforms for studying intriguing low-temperature superconducting properties.
Regarding the symmetry of the superconducting gap, contradicting findings from
several experiments have been made for LaRu$_{4}$As$_{12}$ and its
isoelectronic counterpart, LaOs$_{4}$As$_{12}$. In this vein, we report
comprehensive bulk and microscopic results on LaOs$_{4}$As$_{12}$ utilizing
specific heat analysis and muon-spin rotation/relaxation ($\mu$SR)
measurements. Bulk superconductivity with $T_C$ = 3.2 K was confirmed by heat
capacity. The superconducting ground state of the filled-skutterudite
LaOs$_{4}$As$_{12}$ compound is found to have two key characteristics:
superfluid density exhibits saturation type behavior at low temperature, which
points to a fully gapped superconductivity with gap value of $2\Delta/k_BT_C$ =
3.26; additionally, the superconducting state does not show any sign of
spontaneous magnetic field, supporting the preservation of time-reversal
symmetry. These results open the door for the development of La-based
skutterudites as special probes for examining the interplay of single- and
multiband superconductivity in classical electron-phonon systems.
