Following the 1973 general election, O'Brien was appointed Minister for Posts and Telegraphs in the 1973–77 Labour-Fine Gael coalition under Taoiseach Liam Cosgrave.
Efter 1973 års riksdagsval, blev O'Brien minister för post och telegraf 1973-77 i Labour Fine Gael koalitionsregering under Taoiseach Liam Cosgrave.