Yes and no, Lucas came up with the basic story for all the films (even though he pinched them from all over the place) he later gave them to other writers to do later draughts eg, Leigh Brackett for ESB (but he largely ditched everything she did still giving her the screenwriting credit). He never hid the fact that other people worked on the screenplays for the OT, in fact he possibly took less credit than he could legitimately claim.

With the PT he took much of the credit/blame for the screenplays when many hidden hands also worked on them.