  We have studied via in-beam $\gamma$-ray spectroscopy $^{196}$Po and
$^{198}$Po, which are the first neutron-deficient Po isotopes to exhibit a
collective low-lying structure. The ratios of yrast state energies and the E2
branching ratios of transitions from non-yrast to yrast states are indicative
of a low-lying vibrational structure. The onset of collective motion in these
isotopes can be attributed to the opening of the neutron i$_{13/2}$ orbital at
N$\approx$112 and the resulting large overlap between the two valence protons
in the h$_{9/2}$ orbital and the valence neutrons in the i$_{13/2}$ orbital.
