From the foot $D$ of the height $CD$ in the triangle $ABC$ perpendiculars to $BC$ and $AC$ are drawn, which they intersect at points $M$ and $N$. Let $\angle CAB=60°$ and $\angle CBA=45°$, and $H$ be the orthocentre of $MNC$. If $O$ is the midpoint of $CD$ find $\angle COH$.