  We study the electronic structure of delafossite PtCoO$_2$ to elucidate its
extremely small resistivity and high mobility. The band exhibits steep
dispersion near the Fermi level despite the fact that it is formed mainly by Pt
$d$ orbitals that are typically localized. We propose a picture based on two
hidden kagome-lattice-like electronic structure: one originating from Pt
$s+p_x/p_y$ orbitals, and the other from Pt $d_{3z^2-r^2}+d_{xy}/d_{x^2-y^2}$
orbitals, each placed on the bonds of the triangular lattice. In particular, we
find that the underlying Pt $s+p_x/p_y$ bands actually determine the steepness
of the original dispersion, so that the large Fermi velocity can be attributed
to the large width of the Pt $s+p_x/p_y$ band. More importantly, the
kagome-like electronic structure gives rise to "orbital-momentum locking" on
the Fermi surface, which reduces the electron scattering by impurities. We
conclude that the combination of the large Fermi velocity and the
orbital-momentum locking is likely to be the origin of the extremely small
resistivity in PtCoO$_2$.
