Tadeuz O'Brian was a Manticoran citizen and an enlisted member of the Royal Manticoran Marine Corps.

In 1900 PD, O'Brian held the rank of Sergeant and was assigned to Third Platoon aboard the light cruiser HMS Fearless during its Mission in the Basilisk System. He fought against the Medusans on their planet in defense of the local Manticoran government compound. (HH1)