Answer:
radius = 6.5
<OAC = 67.38 degrees
Explanation:
It looks like you are assuming 0 is on AB and in fact defines the circumradius of the circle. Further you are assuming (I think) that ABC is a right angle triangle.
Therefore AB^2 = AC^2 + BC^2
AB^2 = 5^2 + 12^2
AB^2 = 25 + 144
AB^2 = 169
AB = 13
AO is therefore 1/2 of 13 = 6.5.
CO is also 6.5 since all radii are equal.
I don't know if you know this but the only way you can solve this is to use the Cosine Law for distance.
AC = 5
OC = 6.5
OA = 6.5
<CAO = ??
CO^2 = OA^2 + CA^2 - 2*OA*CA*Cos(<CAO)
6.5^2 = 6.5^2 + 5^2 - 2*5*6.5*Cos(<CAO)
42.25 = 42.25 + 25 - 65*Cos(<CAO)
0 = +25 - 65*Cos(<CAO)
-25 = -65*Cos(<CAO)
0.3846 = Cos(<CAO)
<CAO = cos-1(0.3846)
<CAO = 67.38 degrees.