  We report the results of low-temperature heat capacity measurements of the
pnictide oxide superconductor BaTi$_2$Sb$_2$O doped with sodium. The
temperature and field dependent heat capacity data are well described by a
single-gap BCS theory. The estimated values for the normal state Sommerfeld
constant, the heat capacity jump at $T_c$, and the electron-phonon coupling
constant are in favor of a conventional weak coupling superconductivity
mediated by electron-phonon interaction. The results are discussed with regard
to and compared with recent first principle calculations.
