What does O'Brien mean?
O'Brien \o-brien, ob-rien\ as a name for boys is of Celtic, Irish and Gaelic origin, and the meaning of O'Brien is "high, noble". O'Brien is a derivative of Brian (Celtic, Irish, Gaelic): could also mean "strength". Not widely used as a name for babies.
O'Brien is a rare first name for men. O'Brien is an equivalently unique surname too for both adults and children. (2000 U.S. Census)
Below is a graphed comparison of the usage of O'Brien and corresponding names. Brain, Brian (#168 last year), Brien, Bryan (#148), Bryant (#581) and Bryon are the prominent related forms of O'Brien (#unlisted) ranked in the Top 1000. These boy names were at the top of their popularity during the years 1970-1979 and have become significantly less popular since, with the forms Brain, Bryant and Bryon becoming less trendy. Brian (#168) and Bryan (#148) are two of the more chic boy names here. (2014 Birth Statistics)