The mountainous regions of western Oregon, home to four of the most prominent mountain peaks of the United States including Mount Hood, were formed by the volcanic activity of Juan de Fuca Plate, a tectonic plate that poses a continued threat of volcanic activity and earthquakes in the region. The most recent major activity was the 1700 Cascadia earthquake. (Washington's Mount St. Helens erupted in 1980, an event which was visible from Oregon.)

The Columbia River, which forms much of the northern border of Oregon, also played a major role in the region's geological evolution, as well as its economic and cultural development. The Columbia is one of North America's largest rivers, and one of two rivers to cut through the Cascades (the Klamath River in Southern Oregon is the other). About 15,000 years ago, the Columbia repeatedly flooded much of Oregon during the Missoula Floods; the modern fertility of the Willamette Valley is largely a result of those floods. Plentiful salmon made parts of the river, such as Celilo Falls, hubs of economic activity for thousands of years. In the 20th century, numerous hydroelectric dams were constructed along the Columbia, with major impacts on salmon, transportation and commerce, electric power, and flood control.

Today, Oregon's landscape varies from rain forest in the Coast Range to barren desert in the southeast, which still meets the technical definition of a frontier.

Oregon is 295 miles (475 km) north to south at longest distance, and 395 miles (636 km) east to west at longest distance. In land and water area, Oregon is the ninth largest state, covering 98,381 square miles (254,810 km2).[24] The highest point in Oregon is the summit of Mount Hood, at 11,239 feet (3,426 m), and its lowest point is sea level of the Pacific Ocean along the Oregon coast.[4] Its mean elevation is 3,300 feet (1,006 m). Crater Lake National Park is the state's only national park and the site of Crater Lake, the deepest lake in the U.S. at 1,943 feet (592 m).[25] Oregon claims the D River is the shortest river in the world,[26] though the American state of Montana makes the same claim of its Roe River.[27] Oregon is also home to Mill Ends Park (in Portland),[28] the smallest park in the world at 452 square inches (0.29 m2). Oregon's geographical center is farther west than any of the other 48 contiguous states (although the westernmost point of the lower 48 states is in Washington).