Born in Invorio, near Novara, Ottone was the one of the 6 children of
Uberto Visconti, lord of Massino, and Berta Pirovano. Along with his brother Azzone (later Bishop of Ventimiglia), Ottone was forced to ecclesiastical career by his family. He became canon to Desio and in 1247 chamberlain of the powerful Cardinal Ottaviano degli Ubaldini. Under his patronage, Ottone was appointed by Archbishop of Milan Leone da Perego as his envoy to France in 1252, gaining the trust of Pope Innocent IV, becoming his chaplain.
After the death of Leone da Perego in 1257, Ottone was supported by Cardinal Ubaldini as successor to the Archdiocese of Milan, against Raimondo della Torre's candidacy. This action displeased city's lord Martino della Torre (relative of Raimondo), which claimed the historical autonomy of Milan on Archbishop's appointment. Despite Martino's opposition, Pope Urban IV chose Ottone as new Archbishop on 22 July 1262.
Power struggle in Milan
Ottone's insigne as Archbishop
The Pope's choice don't stop the hostility of Martino della Torre, who occupied Milan Cathedral on August 1262, obtaining the excommunication by papal legate Filippo di Pistoia. This act started a war between Martino's family, the Della Torre or Torriani, and the Visconti. The two families were also politically opposed: the Della Torre were historically Guelphs and allied of Charles I of Anjou, while the Visconti were Ghibellines and exponent of low nobility.
Still in Montefiascone, near Viterbo, where he received Pope's appointment, Ottone marched to Arona on 1 April 1263, where he met several nobles fled by Milan for their opposition to Della Torre. Informed on Ottone's presence in Lombardy, Martino sent his troops to put Arona under siege. Ottone, who occupied the near Rocca of Angera, was forced to surrender on 5 May 1263. Returned in Montefiascone, Ottone lost his powerful ally Urban IV, died on October 1264. Della Torre, however, never get Raimondo formal appointment, and after Martino's death, the once loyal Pallavicino family switched side to Visconti, scheming the assassination of Paganino della Torre, podestà of Vercelli, on January 1266. In response, the new lord of Milan Napoleone della Torre executed 53 nobles, suspected of conjuring. This vicious act undermined Della Torre's grab on Milan, aggravated by Pope Gregory X, a Visconti of Piacenza, who in 1273 confirmed Ottone Visconti as legal Archbishop of Milan. Napoleone della Torre reacted exiling all noble families who don't supported him, causing the formation of an émigré coalition in Novara and Pavia. Using their financial and military support, Ottone's nephew Teobaldo Visconti led an army in Vercelli, occuping Castelseprio. Defeated by Torriani's forces, Teobaldo fled to Lurate, near Como, but after a battle in Gallarate, his last forces were defeated and he was beheaded by Napoleone della Torre in 1276. Ottone, returned in Lombardy in the same year, recruited his supporters near Desio, where he was canon, and after a bloody battle on January 1277, Visconti emerged victorius. Napoleone della Torre was imprisoned and tortured to death in Castel Baradello, while his brother Francesco was executed after the battle. Ottone entered in Milan on 22 January 1277, becoming the first Visconti de facto ruler of the city.
Rule and final years
Became Lord of Milan at age 69, Ottone tried to strengthen his family's power on city and Lombardy. The years of his rule weren't paceful: Della Torre still claimed the signoria (Italian for "lordship") and conquered Lodi and Castelseprio, ruling as rogue power in the region between Adda and Ticino rivers. Ottone seek the support of his ally William VII, Marquess of Montferrat, who demanded the city's government as Capitano generale (general-captain), granted to him in 1278. After William VII's side switch in 1281, Ottone re-acquired full powers on the city, and in the battle of Vaprio d'Adda of 1281, his forces defeated Cassone della Torre, who was killed in the battle, and Raimondo, elder claimant to Milan Archdiocese, who fled to Friuli.
In 1287, the old Ottone appointed his grand-nephew Matteo I Visconti as new Capitano generale, left to him political affairs. On May 1288, Matteo Visconti also gained the title of Imperial vicar by Rudolph I of Germany, who avoided so to intervene directly of Lombard political situation while still nominal ruler of Milan. In his final years, Ottone never get involved in Milan politics, concentrating only of religious affairs. He died on 8 August 1295, at age 88, in the Abbey of Chiaravalle, obtaining the burial in Milan Cathedral.