1) Since OA is perpendicular to DAE, you can calculate the size of angle(BAO)
[In the following statements, the reason that the triangles are isosceles is because
two of their sides are radii.]
2) Since triangle(BAO) is an isosceles triangle and you know the size of angle(BAO),
you can calulate the size of angle(OBA) and then the size of angle(BOA).
3) Since triangle(BOC) is an isosceles triangle and you know the size of angle(CBO),
you can calculate the size of angle(BCO) and then the size of angle(COB).
4) Since you know the sizes of angle(BOA) and angle(COB),
you can calculate the size of angle(COA).
5) Since triangle( COA) is an isosceles triangle and you know the size of angle(COA),
you can calculate the size of angle(OAC).