MathNet 2018-part-06 — 181 problems (EN–VI) Question 127

Given is circle $\Gamma$ with center in point $O$ and diameter $AB$. $OBDE$ is a square, $F$ is the second point of intersection of $AD$ and circle $\Gamma$, $C$ is the middle of the segment $AF$. Find the value of the angle $OCB$.