Ottavio Barsanti was born in Pietrasanta, Tuscany, Italy, probably in 1825 or 1826, the son of Lorenzo Barsanti, a farmer, and his wife, Maria Silionni. His gravestone in Sydney records him as J. O. Barsanti but his baptismal name is not known. He may have received the name Ottavio (Octavius) when he joined the Observant branch of the Franciscan order of priests as a novice. He received the degree of doctor of divinity and became a professor at Assisi and in Rome. In 1860 he was appointed superior of a party of eight Franciscans who were to accompany Bishop J. B. F. Pompallier on his return to New Zealand.
Barsanti arrived in Auckland on 30 December 1860 on the General Teste. Within months of his arrival he had crossed swords with Pompallier over the question of the leadership of the Franciscan group. Pompallier sent Joseph Garavel, a French priest, to work with the Franciscans in the Māori mission in North Auckland. Barsanti claimed that this was tantamount to putting Garavel in charge of the Franciscans. Pompallier had given the Franciscans St Mary's College in Takapuna to use as a friary, and towards the end of 1861 Barsanti objected to its location on the northern shore of the Waitemata Harbour. Without the bishop's permission he moved the group to a rented house in central Auckland. When Barsanti was told by James McDonald, the vicar general, that this action incurred an automatic suspension he hit McDonald, causing him to drop the chalice he was carrying. This was an even graver offence, which carried an automatic excommunication. Pompallier held an official inquiry into the affair, but later lifted both the suspension and the excommunication. However, he relieved Barsanti of the superiorship of the Franciscans.
Barsanti left New Zealand early in 1866 without the permission of either the bishop or his own superiors. He went to Australia and was accepted into the Sydney archdiocese. He incurred Archbishop J. B. Polding's displeasure in 1869 and spent the next three years in 'exile' at St Patrick's Cathedral in Melbourne. On 21 May 1875 Barsanti was suspended from the exercise of priestly functions in the Sydney archdiocese after several warnings. His misdeeds included failure to perform his regular duties, drunkenness, verbal bullying, and at least one case of physical assault. Unabashed, in 1878 he recommended himself to be appointed bishop of Auckland, saying that an Italian bishop was needed to break 'the monotony of Irish abuses'. Between 1876 and 1883 Archbishop Roger Vaughan accumulated a pile of testimonies against Barsanti, supporting the view that he was 'a scandal and a terror to the people' and had 'made a house of the Devil wherever he lived.' In early May 1884 he was reported to be 'settling down' as assistant priest in Newtown, a Sydney parish. He died on 23 May 1884 in St Vincent's Hospital, Sydney. The administrator of the archdiocese wrote to Rome that there had been no time to administer the sacraments but he hoped that Barsanti had died in a state of grace.
On the basis of his five years in New Zealand Barsanti wrote a book about 'the Protestants among the savages of New Zealand, or the history of Pai Mārire' ( I Protestanti tra i selvaggi della Nuova Zelanda, ossia storia del Pai Mārire ), which was published in Turin in 1868. It is clear from the book that Barsanti had no personal contact with the followers of Pai Mārire and very little with any other Māori. He gained information on the progress of the wars and the spread of Pai Mārire from the newspapers of the day as well as from Jean Grange and Joseph Garavel, two priests who had close contact with Pai Mārire. The book is not a history of Pai Mārire, however, but an expression of Barsanti's view of the reasons for such a movement. Pai Mārire was a convenient weapon with which to attack Protestant heresy, the real cause, according to Barsanti, of the troubles afflicting New Zealand. It was only too true, he wrote, that the colours in which Protestantism had been painted were 'not black enough to emphasise its ugliness and deformity'. He considered Pai Mārire 'a formal protest against the teaching of the Evangelicals of London', a natural product of the Protestants' fundamental error, in his view, of allowing their followers to read and interpret the Bible for themselves, and an expression of Māori hatred of the English. He objected to the term 'rebel' being applied to Māori because they refused to subject themselves to a foreign ruler and a heretical religion.
Barsanti's views illustrate the religious intolerance of the time and the situation of an Italian priest, isolated by culture, language and religion both from Māori and from the bulk of the settler population. He was an intelligent and well-educated man, but he was also self-willed, quick-tempered and prone to violence. His pious writing may have been some kind of penance for his outlandish behaviour.