Abstract: In order to enhance speed and quality of OODA loop, assessment is added into OODA loop, and An air defense systemof-systems based on OODA-A is presented. Assessment of battlefield situation and combat effectiveness is done after OODA loop in OODA-A loop, including target maneuver assessment, target information conflict assessment, target damage assessment, missed shot and overshot assessment and combat cost assessment. The next move is determined by assessment results and the assessment results are fed back to nodes in OODA-A loop, therefore the combat capability of nodes in OODA-A loop is improved and the speed and quality of OODA loop is enhanced. The combat timeliness of OODA-A loop is simulated with typical air defense combat cases. The simulation results show that the combat timeliness of OODA-A loop is superior to OODA loop. It has important guiding significance to the improvement of combat capability of air defense system-of-syetems.