This Ph.D. thesis presents a comprehensive analysis of Eugene O’Neill’s transformative role in American literature, establishing him as the primary trendsetter of modern American drama. Before O’Neill, American theatre was largely dominated by superficial melodramas and European imitations; however, through his innovative techniques and profound philosophical inquiries, O’Neill introduced a new era of serious, tragic, and psychologically complex drama. The research examines his stylistic evolution from the gritty realism of his early sea plays to the radical expressionism of "The Hairy Ape" and "The Emperor Jones," and finally to the masterful domestic realism of "Long Day's Journey into Night." Central to this study is an exploration of O’Neill’s preoccupation with the "tragedy of the human soul," influenced by Greek drama, Nietzschean philosophy, and Freudian psychology. The thesis discusses how O’Neill utilized groundbreaking stage devices—such as masks, interior monologues (asides), and symbolic sets—to externalize the subconscious struggles and existential alienation of his characters. By deconstructing O’Neill’s portrayal of the marginalized and the dispossessed, the study highlights his critique of the American Dream and the pursuit of spiritual belonging. The research argues that O’Neill’s legacy lies in his ability to give American drama a voice that was both uniquely national and universally human. The study concludes that O’Neill remains the foundational figure upon whom subsequent playwrights like Arthur Miller, Tennessee Williams, and Edward Albee built their work, forever marking him as the architect of the modern American stage.

Keywords: Eugene O’Neill, Modern American Drama, Expressionism, Existentialism, Realism, Psychoanalysis, Literary Trendsetter.