Mark,

Dr. Jack and W. Owen did a great job of explaining OD/OPP and human COG, as well as provide some historical context, so have a look above.

With regards to Hooah, I usually use it in an affectionate manner, when showing approval or excitement or gearing up to go do something fun. Hooah can also mean "yes" and a number of other different things. I just love my Army...

Be advised that Hooah in the US Army takes a different meaning than OORAAHH in the Marine Corps - they use it more specifically when given a task/mission and it is more of a response that the task is understood and let's go get 'em!

Cheers,
Rach