Depends
on whether the O/B is on the transom or on a bracket, and how heavy it is. The flexing loads imposed by an O/B hanging off the back when going over bumps is enormous. If the motor is clamped on to a well-reinforced transom, and is relatively light (under 9.9 hp), then you are probably OK. If in any doubt, err on the side of caution!! I have seen stress cracks from towing with the motor attached.