AD = DC = 4 cm
MC = DC/2 = 2
∠ACD = 45°
AC = AD / sin(45°) = 5.656854249
∠BMC = tan-1(BC/MC) = 63.435°
∠MOC = 180 - 45 - 63.435 = 71.565°
Let F be the foot of altitude from C to MO
CF = sin(∠BMC) * MC = 1.788854382
OC = CF / sin(∠MOC) = 1.885618645
OA = AC - OC = 3.771235604
The ratio of OC to OA = 1.885618645 / 3.771235604 = 0.5 or 1/2