2013 JBMO Shortlist G1

Let ${AB}$ be a diameter of a circle  ${\omega}$ and center ${O}$ ,  ${OC}$ a radius of ${\omega}$ perpendicular to $AB$,${M}$ be a point of the segment $\left( OC \right)$ . Let ${N}$ be the second intersection point of line ${AM}$ with ${\omega}$ and ${P}$ the intersection point of the tangents of ${\omega}$ at points ${N}$ and ${B.}$ Prove that points ${M,O,P,N}$ are cocyclic

posted in aops here

my solution 
without words