(also Rassa), a river in Minsk and Gomel’ oblasts, Byelorussian SSR, a right tributary of the Ptich’ River of the basin of the Pripiat’ and Dnieper rivers. It measures 151 km long and drains an area of 3,580 sq km.
Flowing along the northern border of the Poles’e region, the Oressa is fed by various sources, primarily snow. The mean flow rate 12 km from the mouth is 15.9 cu m per sec. The river freezes between November and February, and the ice breaks up between March and the first half of April. Provided with a canal system, the Oressa has a number of drainage canals in its basin, and the Liuban’ Reservoir is situated in the upper course. Timber is floated on the river. The city of Liuban’ is on the Oressa.