O'Brien may refer to the following:

• O'Brien (Marine) - A Marine who fought in the Fall of Reach.

• O'Brian (Another Day at the Beach) - The ODST from Another Day at the Beach.

• O'Brien (The Babysitter) - An ODST in The Babysitter episode of Halo Legends.

• William O'Brien - The animator at Bungie Studios.

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