MathNet 2014 Question 73

Segment $CD$ is tangent to the circle with center $O$, at $D$. Point $A$ is in the interior of the circle, and segment $AC$ intersects the circle at $B$. If $OA = 2$, $AB = 4$, $BC = 3$, and $CD = 6$, find the length of segment $OC$.