Dan Ford has a long post on Boyd and the OODA loop over on the Warbird Forum.  

He makes some interesting observations on how Boyd’s thinking about the OODA “loop” evolved over the years.  In particular, he documents that Boyd was still describing the OODA “loop” as a cycle as late as 1991.  My personal feeling is that Boyd felt it was just easier for people to grasp if he explained it one step as a time.  On the other hand, we’re still living with the problems this approach causes.