You asked the distance of oc, so
Settings; The AO distance is X.
BA= 12 AO=X? BO should be equal to ba+bo =12+x.
The triangle OBC is an isosceles triangle.
OC=BO= 12+X
Pythagorean theorem shows that OC=√3OA=√3X.
The equation listed is 12+X=√3X.
The solution is X= 17.