O'Brien may refer to the following:
- O'Brien (Heian) - An ODST in The Babysitter episode of Halo Legends.
- O'Brien (Marine) - A Marine who fought in the Fall of Reach.
- O'Brian (ODST) - The ODST from Another Day at the Beach.
- William O'Brien - An animator at Bungie Studios, also known as Bill O'Brien.
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