Yesaul, osaul or osavul (Russian: есау́л, romanized: yesaul, Ukrainian: осаву́л, romanized: osavul) (from Turkic yasaul - chief),^[1] is a post and a rank in the Russian and Ukrainian Cossack units.

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The first records of the rank imply that it was introduced by Stefan Batory, King of Poland in 1576.

There were different yesaul posts and ranks in Cossack Hosts in Imperial Russia:

• Генеральный есаул (generalny yesaul) - General Yesaul
• Походный есаул (pokhodny yesaul) - Campaign Yesaul
• Войсковой есаул (voiskovoy yesaul) - Army Yesaul
• Полковой есаул (polkovoy yesaul) - Regimental Yesaul
• Артиллерийский есаул (artilleriysky yesaul) - Artillery Yesaul
• Сотенный есаул (sotenny yesaul) - Company Yesaul (commander of a sotnia)
• Станичный есаул (stanichny yesaul) - Yesaul of a stanitsa

In Ukraine of the 17th and 18th centuries, an osaul was a military and administrative official performing the duties of aide-de-camp.^[2] The head of state, hetman, would appoint up to two osauls known as a General Osaul. There also was a Regimental Osaul as well as Company Osaul, with each regular cossack regiment and company except artillery having two of each. Beside them there were osauls under special assignments, one of them serving for General Obozny (quartermaster) who performed duties of a chief executive and was the second in importance after the Hetman.

In the ukrainian folk song "Zadumaly Bazylevtsi" (Задумали Базилевці) about the anti-serfdom uprising in Turbaii (1789—1793), the wife of the Bazylevskyj family sends Yesauls to take care of the serfs, but they are met with unexpected resistance. The serfs end up invading the Bazylevskyj residence and murdering the land owners.^[3]