Points $A$ and $C$ lie on a circle centered at $O$, each of $\overline{BA}$ and $\overline{BC}$ are tangent to the circle, and $△ABC$ is isosceles with $\angle ABC=120^{\circ }$. The circle intersects $\overline{BO}$ at $D$. What is $\frac{BD}{BO}$?