Osman Pazvantoğlu (1758–27 January 1807) was an Ottoman mercenary, the de facto Vizier of the Sanjak of Vidin after 1794 as a rebel against Ottoman rule, amnestied and recognized in 1799 as pasha by the sultan, continuing rebellion the next year. Initially serving as a mercenary in Wallachia, he wrested the Sanjak of Vidin and ruled in defiance of the Ottoman sultan, and fought other Ottoman pashas and made incursions into the Sanjak of Smederevo, Wallachia, and Serbia during the First Serbian Uprising.

Osman's grandfather Osman, who was of Bosniak descent from Tuzla, was a night watch guard or shop guard in Plovdiv, Adrianople and Vidin, as the surname relates (pazvant, "watcher"), that participated in the Russo-Turkish War (1735–1739) and distinguished himself in an attack in Krajina (now eastern Serbia). For this, he received property from the sultan and lived in Vidin. In Vidin, his son Omar Pazvant-zade (Osman's father) was born. According to François Pouqueville, grandfather Osman was a bandit who was executed by impalement at Pristina, and it was Omar who participated in the war. Omar is mentioned in 1764 as having villages, chiftliks, water mills, meadows, wineyards, and more, in the Vidin nahiya, and was the Janissary agha of the 31st orta. Omar was temporarily removed from Vidin, and served in the Belgrade Fortress.

Pazvantoğlu is mentioned as quarreling with his father, then making peace on the interference of the Vidin population. The two joined forces and together removed their rivals in the area, which led to unrest in late 1787. The unrest led to Sultan Abdul Hamid I issuing a firman (decree) to Mehmed Pasha of Vidin and Abdi Pasha of Belgrade to get rid of the rebellious Omar and Osman and protect the cities from their violence. Abdi Pasha was caught up with the threat of an Austrian invasion, while Mehmed Pasha set out and managed to disperse the Vidin rebels, forcing them to take refuge in Krajina. From Krajina, some of them went into Serbia and Bosnia, while Omar and Osman went to Niš and then found refuge with Memiş Pasha at Ada Kaleh.

The Austro-Turkish war began in February 1788, and Orșova was lost, with Memiş Pasha tasked to return it. He asked Wallachian prince and Ottoman vassal Nicholas Mavrogenes to give "courageous" Omar Pazvant-zade "who was denigrated with slander" an army to attack Orsova from Černec. Omar fled Černec with his 17 men and ahead of an Austrian incursion on 18 April, and was captured and executed on the order of Mehmed Pasha of Vidin. Osman had in the meantime joined Mavrogenes' army and fought in Wallachia. He decided to avenge his father and in 1789 took a band which surprise attacked Vidin and forced Mehmed Pasha to give up the garrison, but remained alive with his small entourage after showing the Sultan's ferman on his father's execution. When Belgrade was besieged in July 1789, Osman left Vidin and joined serasker Yusuf Pasha at Kladovo. Osman was defeated by an Austrian Free Corps unit at the Morava and retreated, and after the Ottoman reconquest of the Danube points, he returned to Vidin by autumn 1790.

Osman befriended the Greek Rigas Feraios while in Wallachia, when Feraios became the clerk of Mavrogenes. When Pazvantoğlu disobeyed Mavrogenes on one occasion, he was saved from reprisals through Feraios' intervention.

In springtime 1792, Osman is mentioned as a Janissary leader in Vidin. Despite the end of the war, soldiers remained on the terrain and became bandits and robbers, employed as mercenaries to Turk lords in the Balkans, becoming kircali. The other disorderly group present in the Balkans were the expelled Janissaries from the Belgrade Pashalik, of which the majority gathered in Vidin. In 1792, the Austrians tried to have the 400 prisoners of war in Vidin released, but the Vidin Pasha explained that the Vidin Turks under Pazvantoğlu refused to accept this. In May 1792 a ferman ordered for the release of all Austrian prisoners in Vidin, but this was not done, and another ferman sentenced Pazvantoğlu and his two friends to expulsion to Kars due to the rebellion against the former Vidin Pasha and their gathering of "rebels of the Belgrade environs to provoke unrest in the empire". Pazvantoğlu paid the new Vidin Pasha off, to remain in Vidin with his followers, and even received the right to collect the tax on the gypsies in the Sanjak of Vidin, which showed his growing reputation. The expulsion to Kars was terminated by the Sultan, and Pazvantoğlu's entourage was left to live in Vidin "only in peace". The Belgrade Janissaries, who had been expelled earlier and now revolted and took the Belgrade town, were expelled in November 1792.

In 1793, he undertook a military expedition to the Sanjak of Smederevo but was soundly defeated by the Serbs in Ottoman service at the Battle of Kolari.

Having gathered a large army of mercenaries, he revolted against the Ottoman sultan Selim III, and, acting as an independent ruler, minted his own coins and had diplomatic relations with foreign states (including the French Republic). In 1798, he held territories which spread from the Danube to the Balkan Mountains and from Belgrade to Varna.