I have 2002 Yukpn Denali XL. There is a OC code that appears in the rear view. What is the cause of this and how can I fix it?

The "OC" indicates your compass is "Out of Calibration". Below is the proceedure to recalibrate your compass.

Compass Calibration
The compass may need calibration if:

After five seconds, the display does not show a
compass heading (N for North, for example), there
may be a strong magnetic field interfering with the
compass. Such interference may be caused by a
magnetic antenna mount, magnetic note pad holder
or a similar magnetic item.

The compass does not display the correct heading
and the compass zone variance is set correctly.
In order to calibrate, the letters CAL must be displayed
in the mirror compass windows. If CAL is not
displayed, push in the COMP button for approximately
eight seconds or until the letters CAL are displayed.
The compass can be calibrated in one of two ways:

Drive the vehicle in circles at 5 mph (8 km/h) or less
until the display reads a direction, or

drive the vehicle on your everyday routine and after
several turns the compass will become calibrated and
will display a direction.

Good luck.