HOOAH, OORAH, and HOOYAH

We were watching Jeopardy! and the question was regarding the birthdate of the U.S. Marine Corps. After answering, Gerald let out a whooping "HOOOAH". I asked, "I wonder where that originated?" I asked Les, "How do you think it's spelled?"

See the definition from The Wise Geek:

“Hooah!” is a slang phrase used by members of the U. S. military, and although its exact origin is not clear, it is typically thought to come from the acronym HUA, which stands for for “heard, understood, acknowledged.” The phrase is generally used in response to a statement or order from a military superior. “Hooah!” also is often used by members of the U. S. military as a morale-boosting cheer, and its broad definition has even been described as meaning "everything and anything but no."

After a little research, I learned that the Army uses "HOOAH", the Marines "OORAH" and the Navy and Coast Guard "HOOYAH!".