Due to the symmetry of the rhombus; AC = 2OC and BD = 2OD. Hence, we can say that OC is 4cm and OD is 3cm. Then we can say that sin(OCD) = OC/CD [CD = sqrt(OC^2 + OD^2)].
At the end, did you mean sqrt(sin(ocd)) or sin(sqrt(ocd)). Either way you can find angle OCD by doing arcsin(OC/CD).