Osiek Jasielski [ˈɔɕɛk jaˈɕɛlski] is a village in Jasło County, Subcarpathian Voivodeship, in south-eastern Poland.

The defensive settlement of Osiek probably existed since early Middle Ages, guarding southeastern corner of Lesser Poland.

In 1365, King Casimir the Great decided to found here a town, together with a castle, with the purpose of defending Poland from possible Hungarian raids.

Osiek was the seat of a starosta, and the newly established town was famous for its fairs, which took place twice a year, on 28 April and 13 July.

The policemen then led the rest of their captives, including all children and the elderly, into the forest, where they massacred them with machine guns and buried them in pits.