The 21-year-old made 13 appearances in all competitions for the Exiles while on loan at the Rodney Parade club earlier this season, scoring one goal.
He previously spent three months with Port Vale in 2015, playing five games for the League One side.
O'Sullivan is yet to start a Bluebirds first-team match, but has made three substitute appearances for Cardiff.
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Newport County have re-signed Cardiff City midfielder Tommy O'Sullivan on loan until the end of the season.