OODA stands for Observation, Orientation, Decision, Action...

...the enemy makes a mistake... He cannot decipher what you are doing, and as he becomes increasingly cut off from the reality of the battlefield, he reacts to things that are not there, and his misreactions spell his death.

Boyd saw this theory as having application to all forms of warfare...

If the pilot is too slow and conventional in his thinking, he will find himself falling further and further behind in the loops. His ideas will not keep pace with reality.