  The layered delafossite compound AgNiO$_2$ with $2H$ stacking symmetry
undergoes a structural metal-to-metal transition at $T_{\rm S}\sim 365$ K. It
has been described in the past as a charge-ordering transition, where local
$S=1$ spins are formed on part of the Ni sites. By means of first-principles
many body calculations, we show that the transition is in fact a site-selective
Mott transition on the Ni sublattice with only minor charge differentiation.
Key to this finding is the uncovering of ligand-hole physics, rendering a
Ni$^{2+}$ instead of a formal Ni$^{3+}$ oxidation state, in conjunction with
strong local Coulomb repulsions.
