See sibling comment for a good definition. OODA is the repetitive loop of observing, understanding, planning and executing. The military talk about getting inside the enemy's OODA loop, by which they mean having a faster decision-making cycle.
But back in the network-enabled-capability day, they also used to talk about self-synchronising units operating on the cusp of tactical anarchy, but then Afghanistan happened and they had a collective sense-of-humour failure regarding these future operational concepts.