2. The given circle is the incircle of the triangle.
3. For convenience I've labeled the side OA = a, OB = b and AB = c.
4. Divide the triangle into three triangles with the radius of the circle as its' height. (see attachment)
5. Now you know:
6. Thus you have to solve for (a,b,c) the system of equations:
7. I'll leave the rest for you.