The 'OH' indicator means the fireplace has overheated — specifically, that its thermal limit safety sensor has detected excessive heat, typically due to a lack of airflow. To fix it, turn the unit off and unplug it, then allow it to cool down for 15–30 minutes before restarting. If the error repeats, it may signal an airflow obstruction, a failing thermal switch, or a control board issue that requires professional inspection.