Boyd wrote that orientation is the most important part of the OODA loop (Organic Design, 26).
Far be it from me to contradict the master, but a good exercise for you would be to explain why each component of the OODA “loop” is the most important. I’ll get you started: Without action, who cares what the rest of your OODA “loop” is doing? As Boyd himself began the “Abstract”:
To flourish and grow in a many-sided, uncertain and ever-changing world that surrounds us suggests that we have to make intuitive within ourselves those many practices we need to meet the exigencies of that world. (emphasis added)
Because action is the only thing that opponents, competitors, allies, the undecided, and customers see.
Looked at this way, the OODA “loop” is a device for representing how we evolve our capability for putting actions into the world. As I explain in “Boyd’s Real OODA Loop,” if you use the right one, the one Boyd put into his last briefing, you can generate any pattern of actions that your imagination and preparation allow.
This, from a recent conference. The circular pattern in blue just indicates that these links are operating and does not imply that they are firing in sequence (i.e., waiting on each other). Click for a larger view.
My paper, and Boyd’s “Abstract” are available from the Articles page in the menu, above.
Slightly East of New 
“Because action is the only thing that opponents, competitors, allies, the undecided, and customers see.”
Now there is a thought. Can you actually “see” action, because it is like looking at a wooden table and not understanding that everything that you are looking at is moving and the greatest quantity you are looking at is empty space. It might be empty space in a field, but it is sure not anything closely related to what you are looking at in action.
What you are actually looking at is something that has an advantage in being oriented towards looking like a table.
And what your opponent, competitor, allies, the undecided, and customers “see” is a glimpse of the future and past as the movement and space Orients into looking like a table.
In other words, action is only important in giving them a glimpse of where you have been and where you are going (both “ends” of strategy), but it is not telling you the energy nor power available in the action. To me that is the most important thing in understanding an OODA loop. Energy is force at a distance, which is how the loop Orients itself.
Power is the movement of that energy, and, as you suggest, you can “see” that movement in Action. All power is created on speculation, and that speculation builds in Decision before it is released for “show” in Action.
The narrative of the energy and power available is all told in Orientation, and comes from an Observation of the environment your opponents, competitors, allies, the undecided, and customers see.
Which to me means that Observation is the most important, because it is all seeing, if you have the wisdom 🙂