The Oyat (Russian: Оять) is a river in Babayevsky District of Vologda Oblast and Podporozhsky and Lodeynopolsky Districts of Leningrad Oblast of Russia, a major left tributary of the Svir (Lake Ladoga basin). The length of the Oyat is 266 kilometres (165 mi), and the area of its drainage basin is 5,220 square kilometres (2,020 sq mi).

The source of the Oyat is Lake Chaymozero in the western part of Babayevsky District. The Oyat flows to the northwest and enters Leningrad Oblast. In the village of Shandovichi it turns north. Upstream of the selo of Vinnitsy the Oyat accepts the Tuksha from the right and sharply turns southwest. It enters Lodeynopolsky District and in the selo of Alekhovshchina turns northwest. The mouth of the Oyat is located in the selo of Domozhilovo. Much of the valley of the Oyat in Leningrad oblast is populated.

The drainage basin of the Oyat includes the southern parta of Podporozhsky and Lodeynopolsky Districts, the areas in the west of Vytegorsky and Babayevsky Districts of Vologda Oblast, as well as minor areas in the north of Tikhvinsky District of Leningrad Oblast. There are many lakes in the basin of the Oyat, the biggest of them being Lake Savozero.