This explanation is compliments of the United States Marine Corps League. I will later give the Native American legend version.

Oorah is a spirited cry common to United States Marines since the mid-20th century. It is comparable to the “Huaa” (heard, understood, and accepted – pronounced Hooah) cry used in the Army, but is probably more commonly used among Marines than “Huaa” would be in the Army. It is most commonly used to respond in the affirmative to a question, to acknowledge an order, or as an expression of enthusiasm.

The 1st Amphibious Reconnaissance Company, FMFPAC can be credited with the introduction of “Oorah!” into the Corps in 1953, shortly after the Korean War. Recon Marines served aboard the submarine USS Perch, ASSP-313, which was a WWII diesel submarine retrofitted to carry Navy UDT and Recon Marines. As is commonly depicted in war movies, whenever the boat was to dive, the PA system would announce “DIVE! DIVE!”, followed by the sound of a horn: “AARUGHA!”.

In 1953 or 1954, while on a conditioning run, one of the 1st Amphibious Recon Marines imitated the “Dive” horn sound “AARUGHA!” as part of the cadence, and it naturally became a part of the Recon cadence while on runs, and thereafter infiltrated Recon Marine lexicon. Over time, “AARUGHA!” morphed into the shorter, simpler “Oorah!” Today, the official Marine Corps Training Reference Manual on the history of Marine Recon is titled “AARUGHA!”

Former Sergeant Major of the Marine Corps John Massaro, while serving as the company Gunnery Sergeant of 1st Force Amphibious Recon in the late 1950s, accquired “Oorah!” and took it with him when he went to serve as an instructor at the Drill Instructor school at Marine Corps Recruit Depot San Diego. He there passed it on to the Drill Instructor students and they, in turn, passed it on to their recruits.

Owing to its relatively recent origins, it is less common for Marines who served in Vietnam or earlier to be familiar with “Oorah!”, but most post-Vietnam Marines will have learned it throughout their careers.

Several apocryphal origins of “Oorah!” exist. One has it that the term is Turkish for “Kill”. How a Turkish word entered the Marine Corps is not typically explained. In fact, the Turkish word for “kill” is “öldürmek”. However, the Russian battlecry of “Urrah!” can supposedly be traced to the Turkish word for “Kill.”

A shortened version of “Oorah!” can come out as a short, sharp, monosyllabic gutteral “Er!”

Retired Marine Corps Gunnery Sergeant, Drill Instructor and Hollywood actor R. Lee Ermey is known for using “Oorah!” frequently on the History Channel program Mail Call, which he hosts.

“Oorah” and “Hooah” may also be variations of the earlier “Hurrah”, which was a common shout used by English-speaking soldiers in the past.