O'Sullivan's poetry is metaphysical because it delineates the mystic path of the pilgrim/persona, the dark nights of the soul, and the final harmony that prevails after the union of the soul with the Divine. This mystical experience forms the subject matter of the two volumes, and is conveyed by a central image depicting God as a Divine Lover that woos the human soul/beloved. The courtship and marriage of the Lover and the Beloved is described both in spiritual as well as erotic terms. O'Sullivan ends this highly personal drama with the desired marriage between the feminine soul, and masculine God. To O'Sullivan, the Holy Marriage of the Bride and the Bridegroom proved the most important cause in man's life. By fusing Catholic views, medieval conventions, and romantic forms, O'Sullivan succeeded in conveying his ideas effectively.