It is in western Siberia, and with its tributary the Irtysh forms the world's seventh-longest river system, at 5,410 kilometres (3,360 mi).

It is the westernmost of the three great Siberian rivers that flow into the Arctic Ocean (the other two being the Yenisei and the Lena).

[8] The Ob forms 25 km (16 mi) southwest of Biysk in Altai Krai at the confluence of the Biya and Katun rivers.

[9] The Biya has its sources in Lake Teletskoye and the 700 kilometres (430 mi) long Katun in a glacier on Mount Byelukha.

The Ob zigzags west and north until it reaches 55° N, where it curves to the northwest, south of the Siberian Uvaly, at the western end of which it bends northwards, wheeling finally eastwards into the Gulf of Ob, a 1,000-kilometre-long (620-mile) bay of the Kara Sea, separating the Yamal Peninsula from the Gyda Peninsula.

The combined Ob-Irtysh system, the fourth-longest river system of Asia (after Yenisei, and China's Yangzi and Yellow rivers), is 5,410 kilometres (3,360 mi) long, and the area of its basin 2,990,000 square kilometres (1,150,000 sq mi).

The river basin of the Ob consists mostly of steppe, taiga, swamps, tundra, and semi-desert topography.

The Novgorodians were aware of the lands of western Siberia from at least the 11th century, which were designated by the Russian word Yugra.

[13] Until the early 20th century, a particularly important western river-port was Tyumen, located on the Tura, a tributary of the Tobol.

In the lower reaches, the maximum permissible concentrations of petroleum products are exceeded by 9–10 times.

The oxygen content in the water is 4 times lower than normal[16] The Irtysh is the major tributary of the Ob.