On 2 May 1866, he participated in the Battle of Estero Bellaco, a depression of terrain by which the waters of the Paraná River join the waters of the Paraguay River, where the enemy began to delay the allied troops, seeking a better position to fight. On 24 May 1866, he was present at the Battle of Tuyutí, the largest combat in South America, in which he played an important role in commanding the center of the Brazilian army's military device against the Paraguayan attack. He recorded on the Order of the Day #56: "Glory is the most precious reward of the brave." On 15 July 1866, still in Tuyuti, the allies awaited the arrival of the forces of General Count of Porto Alegre, while the enemy whipped daily. Seriously wounded in the battle and dissatisfied with the long period of the parked troop, Osório passed the command of the Brazilian troops to General Polidoro da Fonseca Quintanilha Jordão.