Answer by taking a quote from the following article:

Oklahoma! is the first musical written by the team of composer Richard Rodgers and librettist Oscar Hammerstein II. The musical is based on Lynn Riggs' 1931 play, Green Grow the Lilacs. Set in Oklahoma Territory outside the town of Claremore in 1906, it tells the story of farm girl Laurey Williams and her courtship by two rival suitors, cowboy Curly McLain and the sinister and frightening farmhand Jud Fry. A secondary romance concerns cowboy Will Parker and his flirtatious fiancee, Ado Annie.

Between the world wars, roles in musicals were usually filled by actors who could sing, but Rodgers and Hammerstein chose, conversely, to cast singers who could act. Though Theresa Helburn, codirector of the Theatre Guild, suggested Shirley Temple as Laurey and Groucho Marx as Ali Hakim, Rodgers and Hammerstein, with director Rouben Mamoulian's support, insisted that performers more dramatically appropriate for the roles be cast. As a result, there were no stars in the production, another unusual step. The production was choreographed by Agnes de Mille (her first time choreographing a musical on Broadway), who provided one of the show's most notable and enduring features: a 15-minute first-act ballet finale (often referred to as the dream ballet) depicting Laurey's struggle to evaluate her suitors, Jud and Curly.  The first title given to the work was Away We Go! which opened for out-of-town-tryouts in New Haven's Shubert Theatre on March 11, 1943. Expectations for the show were low; Hammerstein had written six flops in a row, and the show had no star power. Producer Mike Todd walked out after the first act during the tryout and wisecracked, "No legs, no jokes, no chance." But Rodgers and Hammerstein were confident. The New Haven and Boston audiences were enthusiastic, although the reviews were only fair. Of the changes made before the show went to Broadway, two would prove significant: the addition of the show-stopping musical number, "Oklahoma!" and the decision to retitle the musical after that number.  Todd had been wrong; the show opened on Broadway to raves from the critics, sold out, and won a special Pulitzer Prize. Brooks Atkinson wrote in The New York Times that the show's opening number, "Oh, What a Beautiful Mornin'" changed the history of musical theater: "After a verse like that, sung to a buoyant melody, the banalities of the old musical stage became intolerable." The New York Post was the only major paper to give Oklahoma! a mixed review. Its critic felt that while the songs were pleasant enough, they sounded much alike. The show's creativity stimulated Rodgers and Hammerstein's contemporaries and ushered in the "Golden Age" of American musical theatre.

Were the leads known actors or new faces?
As a result, there were no stars in the production, another unusual step.