The Cascade Range, extending north-south, divides Oregon into distinct eastern and western regions, each of which contains a great variety of landforms.

At the state's western edge, the Coast Range, a relatively low mountain system, rises from the beaches, bays, and rugged headlands of the Pacific coast. Between the Coast and Cascade ranges lie fertile valleys, the largest being the Willamette Valley, Oregon's heartland. The two-thirds of the state lying east of the Cascade Range consists generally of arid plateaus cut by river canyons, with rolling hills in the north-central portion giving way to the Blue Mountains in the northeast. The Great Basin in the southeast is characterized by fault-block ridges, weathered buttes, and remnants of large prehistoric lakes.

The Cascades, Oregon's highest mountains, contain nine snow-capped volcanic peaks more than 9,000 ft (2,700 m) high, of which the highest is Mt. Hood, at 11,239 ft (3,428 m). A dormant volcano, Mt. Hood last erupted in 1865. (Mt. St. Helen's, which erupted in 1980, is only 60 mi/97 km to the northwest, in Washington.) The Blue Mountains include several rugged subranges interspersed with plateaus, alluvial basins, and deep river canyons. The Klamath Mountains in the southwest form a jumble of ridges where the Coast and Cascade ranges join.

Oregon is drained by many rivers, but the Columbia, demarcating most of the northern border with Washington, is by far the biggest and most important. Originating in Canada, it flows more than 1,200 mi (1,900 km) to the Pacific Ocean. With a mean flow rate of 250,134 cu ft per second, the Columbia is the 3rd-largest river in the US. It drains some 58% of Oregon's surface by way of a series of northward-flowing rivers, including the Deschutes, John Day, and Umatilla. The largest of the Columbia's tributaries in Oregon, and longest river entirely within the state, is the Willamette, which drains a fertile valley more than 100 mi (160 km) long. Better than half of Oregon's eastern boundary with Idaho is formed by the Snake River, which flows through Hell's Canyon, one of the deepest canyons in North America.

Oregon has 19 natural lakes with a surface area of more than 3,000 acres (1,200 ha), and many smaller ones. The largest is Upper Klamath Lake, which covers 58,922 acres (23,845 ha) and is quite shallow. The most famous, however, is Crater Lake, which formed in the crater created by the violent eruption of Mt. Mazama several thousand years ago and is now a national park. Its depth of 1,932 ft (589 m)—greater than any other lake in the US—and its nearly circular expanse of bright-blue water, edged by the crater's rim, make it a natural wonder.