Oohrah

Oohrah or Oorah is a phrase used uniquely by the UNSC Marine Corps to show excitement or approval in a certain situation.

Summary
Historically, it originated from the United States Marine Corps of the 20th and 21st Centuries, but has apparently been retained as a part of the UNSC Marine Corps culture and phrases.

Halo 2
Oohrah has been heard in Halo 2, in first cutscene of the level Outskirts. It was said by two Pelican pilots in response to Sgt. Johnson's orders.

Halo 3
Oohrah has also been heard in Halo 3, just after the first cutscene of the level The Ark. It was said by ODSTs in John-117's Pelican, seconds before disembarking.

Trivia

 * Oohrah may have been said by Marines in Halo: Combat Evolved, on the levels The Truth and Reconciliation, and The Silent Cartographer just before exiting a Pelican.
 * Oohrah seems to be said only before combat begins, just before jumping from a Pelican dropship into combat, or after receiving favorable orders.