The son of Spoletina Veronica Carosi and of the well-known architect Raffaello Ojetti, a personality of vast culture, he graduated in Law and, together, began as a poet (Landscapes, 1892). He is attracted to a diplomatic career, but is professionally accomplished in political journalism. In 1894 he formed a relationship with the nationalist newspaper La Tribuna, for which he wrote his first services as a foreign correspondent, from Egypt.
In 1895 he suddenly became famous for his essay "Alla scoperta dei letterati" (Milan, 1895), portraits of famous writers of the period written in interviews, a genre that was still in an embryonic state. Written with a style that arises between criticism and reportage, the text is considered, and as such does discuss, a moment of profound analysis of the literary movement of the time. The following year Ojetti held in Venice the conference "The future of literature in Italy", which elicited a large number of comments throughout the country.
His articles become very popular: he writes for Il Marzocco (1896-1899), Il Giornale di Roma, Fanfulla on Sunday and La Stampa. Art criticism occupies most of its production. In 1898 he began working with Corriere della Sera, which continued until his death.
Between 1901 and 1902 he was sent to Paris for the Giornale d'Italia; from 1904 to 1909 he collaborated with L'Illustrazione Italiana: he wrote a column entitled "Next to life", which then renamed "I capricci del conte Ottavio".
In 1905 he married Fernanda Gobba and took up residence in Florence; three years later his daughter Paola was born from the wedding. From 1914 he will permanently live in nearby Fiesole. Instead it is located in the paternal villa of Santa Marinella (Rome), nicknamed "Il Dado", the ideal place to rest, spend your holidays and write your works. He participated as a volunteer in the First World War, also writing the second leaflet, judged more effective than that of D'Annunzio, which was spread in 350,000 copies in the skies of Vienna, in the enterprise of August 9, 1918.
In 1920 he founded his art magazine, Dedalo (Milan, 1920-1933), where he worked on ancient and modern art history. At the end of the decade he inaugurates a new magazine, Pegaso (Florence, 1929-1933). Finally, he launches the literary magazine Pan, founded on the ashes of the previous Florentine experience. Between 1925 and 1926 he also collaborated with La Fiera Letteraria. Between 1926 and 1927 he was director of the Corriere della Sera.
He is one of the signers of the Manifesto of the fascist intellectuals in 1925 and is named Academic of Italy in 1930. He is part of the board of directors of the Italian Encyclopaedia until 1933. Ojetti organizes numerous art exhibitions and gives life to important publishing initiatives, such as the most beautiful pages of Italian writers for Treves and Italian Classics for Rizzoli.
He also collaborated with the cinema: in 1939 he was the adaptation for the first sound edition of I promessi sposi, which formed the basis of the screenplay for the 1941 film by Mario Camerini.
He joined the Italian Social Republic and the liberation of Rome, in 1944, he was removed from the Order of Journalists. He lived the last years in his villa Il Salviatino, in Fiesole, where he died in 1946.