It is formed 8 mi. southwest of Biysk in Altai region, by the confluence of the Biya and the Katun. Both these streams have their origin in the Altai (Sailughem) Mountains, the former issuing from Lake Teletskoye, the latter, 80 mi. long, bursting out of a glacier on Mount Byelukha. The Ob zigzags west and north until it reaches 55° N; thence it curves round to the NW, and again N, wheeling finally eastwards into the Gulf of Ob, a deep (600 mi.) bay of the Arctic Ocean.
The river splits up into more than one arm, especially after receiving the large river Irtysh (from the left) in 69° E. Other noteworthy tributaries are: on the right, the Tom, the Chulym, the Ket, the Tym and the Vakh; and, on the left, the Vasyugan, the Irtysh (with the Ishim and the Tobol) and the Sosva.
The navigable waters within its basin reach a total length of 9300 mi. By means of the Turn, an affluent of the Tobol, it secures connexion with the Ekaterinburg-Perm railway at Tyumen, and thus is linked on to the rivers Kama and Volga in the heart of Russia. Its own length is 3700 km (2260 mi.), and the area of its basin 2,600,000 km² (1,125,200 sq. mi.).
The combined Ob-Irtysh system, the longest river system of Asia, is about 5410 km (about 3362 mi.) long. A system of canals, utilizing the Ket river, 560 mi. long in all, connects the Ob with the Yenisei.
The Ob is ice-bound at Barnaul from early in November to near the end of April, and at Obdursk, 100 mi. above its mouth, from the end of October to the beginning of June. Its middle reaches have been navigated by steamboats since 1845.
Cities along the river include: