In my experience, when it reads 'OC' it is telling you that the two wire circuit to the exterior temp sensor is 'open circuit' (infinite resistance). Either one -or both- of the wires is open somewhere between the display head and the temp sensor, or the sensor itself is open. Or, less common, a solder joint opened up on the circuit board in the display.
And no, it will not affect your mileage. The Engine Coolant Temperature sensor will, as will the Intake Air Temperature sensor, but not the ambient temp sensor.