In 1488, when the city of Forlì was attacked by the Orsi brothers, Checco and Ludovico, the same pair which his mother had hired to kill his father, Ottaviano was taken prisoner by the two along with his sister, Bianca.
While he and his sister were held captive, Ottaviano was kept at a lighthouse in Romagna, watched by Ludovico. However, Ezio Auditore da Firenze, an Assassin, soon came to Ottaviano's aid and killed Ludovico, after rescuing Bianca. With this, Ottaviano ran back to his mother at the Rocca di Ravaldino.
When he was 19, Ottaviano entered into the service of the Florentine Republic as a condottiero, at the request of his mother, Caterina Sforza, who wanted to cement amiable relations with Florence. He commanded 100 men from Forlì, but terminated his contract after only a year, since Florence didn't have the money to pay him.
Despite accounts which described Ottaviano as obese, brainless and under his mother's thumb, Pope Alexander VI, Rodrigo Borgia, offered his daughter Lucrezia to Ottaviano, as Rodrigo decided that those were traits which he could tolerate in a son-in-law, as long as they were the key to controlling the territories of Forlì and Imola. Plus, as his viper of a daughter rarely stayed married to one man for long, with them tending to sign declarations of impotence or just simply disappearing, it would suit the Pope's ambitions quite well.
Wisely however, Caterina denied the marriage, enraging the Pope and leading to his son, Cesare Borgia, initiating an assault on Forlì.
When Caterina was ousted by Cesare and entered her exile in Florence, under her guidance, Ottaviano attempted to convince the successor of Alexander VI, Pope Julius II, to give him back the Lordship of Imola and Forlì. Instead, Ottaviano ended up selling the Riario claim to Julius for a delayed payment, with him losing out in the long term.
- Ottaviano, his siblings, and Petruccio Auditore da Firenze were the only non-adult characters to appear in Assassin's Creed II.
- His face model was recycled in Assassin's Creed: Brotherhood, appearing in the missions Homecoming and Human Cargo.