This is a response to VC 26 Janine 1 ("Pitch in!")...
Oohaah! is a military cultural term(?). It is said more as a grunt and it basically means, "Right on. What a fighting dude."
I have not read backwards on this branch beyond Yumi's card. This card is not quite a response as much as it is a disclaimer or perhaps a confession of sorts.
My impression is that I am most likely the most conservative in this bunch.