While still in their girlhood, Olivia and her sister, the Future Helen Rossetti Angeli (1879-1969), began publishing an anarchist journal, The Torch, in the basement of their family home. Despite their youth, this effort became the nucleus of a prominent anarchist salon which included Peter Kropotkin and Sergei Kravchinski, and their publishing coups included the pamphlet Why I Am an Anarchist by George Bernard Shaw. Years later, using the pseudonym "Isabel Meredith", Olivia and Helen published A Girl Among the Anarchists, a somewhat fictionalized memoir of their days as precocious child Revolutionaries. These adventures were also chronicled by their cousin Ford Madox Ford in his 1931 memoir Return to Yesterday.

A more permanent consequence of this political activity was Olivia's marriage in 1897 to the Italian anarchist and Journalist Antonio Agresti (1866-1926), which led to her emigration from England to Italy. Olivia was to remain there for the rest of her life and she eventually became an Italian citizen. During her first years in Italy she continued with literary activities related to her political activism, including a biography of the Italian Painter and revolutionary Giovanni Costa.

The second phase in Agresti's career began in 1904, when she met the American agricultural reformer David Lubin. A former department store and mail order magnate from Sacramento, Lubin was in Rome seeking a state sponsor for his idea of an international clearinghouse for agricultural statistics. Unable to speak Italian, Lubin hired Agresti as his interpreter and thus began a close collaboration between the two which continued until Lubin's death.

With Agresti's assistance, Lubin's efforts in Italy made history. After gaining the unexpected support of Italy's king Victor Emmanuel III, Lubin's vision became a reality with the 1905 founding of the International Institute of Agriculture, headquartered in Rome. The first modern international organization, it was hailed as a significant forerunner of world government by such luminaries as H. G. Wells and Louis Brandeis. Agresti includes samples of Lubin's correspondence with Wells and Brandeis in her 1922 biography David Lubin: A Study in Practical Idealism.

Following Lubin's death in 1919, Agresti waged a public campaign for close cooperation between the International Institute of Agriculture and the nascent League of Nations, which soon employed her as a member of the staff to Italy's delegation. She continued as a staff interpreter for the League in Geneva from 1922 to 1930. Her last assignment as a professional interpreter occurred in 1945 when, at the personal request of Italy's prime minister, Alcide De Gasperi, she accompanied him to London for meetings of the Council of Foreign Ministers. According to her unpublished memoirs, meetings at which Agresti interpreted included Ernest Bevin and Vyacheslav Molotov.

By 1921 Agresti's early anarchist leanings, further leavened by years of exposure to Lubin's theories concerning cooperative organization of society, had transformed her into an enthusiastic supporter of corporatism and, consequently, of Benito Mussolini's corporatist reorganization of the Italian economy. From 1921 to 1943 she edited the newsletter of the Associazione fra le Società per Azioni, a group then closely allied with the Fascists, and in 1938 co-authored the theoretical work The Organization of the Arts and Professions in the Fascist Guild State with the Fascist Journalist Mario Missiroli.

In 1937, Agresti's editorship of economic journals brought her into professional contact with Ezra Pound, then residing in Italy and writing articles in Italian on economic topics. Thus began a long correspondence between the two which lasted until 1959, the year before Agresti's death. Interestingly, Agresti seems to have been unaware of Pound's fame as a poet, and Pound unaware of Agresti's family ties to Dante Gabriel Rossetti and Ford Madox Ford, two of the most important influences on his poetry, until years into the correspondence. Portions of this correspondence were edited and published in 1998 by the University of Illinois Press.