Oregon's portion of the Interstate Highway System totals 729.57 miles (1,174.13 km).

[1] Transfers of highways between the state and county or local maintenance require the approval of the Oregon Transportation Commission (OTC), a five-member governor-appointed authority that meets monthly.

[2] These transfers often result in discontinuous highways, where a local government maintains part or all of a main road within its boundaries.

Many routes are signed on streets which are maintained by counties and cities, and thus are not part of the state highway system at all, e.g. OR 8, whose eastern- and westernmost portions, Canyon Road and Gales Creek Road, are not actually state highways.

[1][4] The OTC designates the paths of these routes as they follow state highways and local roads;[4] any U.S. Route or Interstate numbers must also be approved by the American Association of State Highway and Transportation Officials (AASHTO).

The initial primary state highway system was designated in 1917,[3] initially consisting of 36 named and numbered highways,[5] including some designated earlier that year by the Oregon State Legislature and others added to the network by the Oregon State Highway Commission, the predecessor to the OTC.

[citation needed] Starting in late 1931, the state took over maintenance of many county "market roads", which became secondary state highways with three-digit numbers;[6] some of these were assigned route numbers in 1935, but many others remained unsigned.

Secondary route numbers, three digits starting with 2, were laid out to generally increase bearing west.

In 2002 and 2003, ODOT decided to assign route numbers to most of the previously unsigned secondary highways.

Two state highways lack route numbers: Century Drive Hwy No.