Oorah is a battle cry common in the United States Marine Corps since the mid-20th century. It is comparable to hooah in the US Army and hooyah in the US Navy and US Coast Guard. It is most commonly used to respond to a verbal greeting or as an expression of enthusiasm.Oorah

What is Semper?

Latin phrase. : always faithful —motto of the U.S. Marine Corps.

Is it OK for a civilian to say oorah to a Marine?

It is oorah, as long as its Marine Corps related. Just say it correctly, and if you need an example watch Jamie Foxx say it in the movie Jarhead. The Army gives a Hoo-uhh (think Scent of a Woman) and the Navy gives a Hooyah.