"Over the Gales Creek County Road from its junction with the Wilson River Highway, OR6, near Gales Creek, southeasterly to its junction with the Nehalem Highway, OR47, in Forest Grove; thence easterly over the Nehalem Highway (common with OR47) to its junction with the Tualatin Valley Highway Spur at College Way in Forest Grove; thence easterly over the Tualatin Valley Highway Spur to its junction with the Tualatin Valley Highway; thence easterly over the Tualatin Valley Highway via Cornelius, Hillsboro, and Beaverton to its junction with the Sunset Highway, US26, near the Washington-Multnomah County Line."
~ ODOT, Descriptions of US and Oregon Routes, March 2007
OR-8 didn't exist at the inception of the Oregon state route numbering system in 1932; the entire Tualatin Valley Highway, from Portland to Forest Grove to McMinnville was numbered OR-47. However, because of the inclusion of secondary state highways into Oregon's highway system, OR-47 is rerouted to a more north-south route (the Nehalem and Mist-Clatskanie Highways, #102 and #110 respectively), and OR-8 is created between Forest Grove and Portland. However, this designation wouldn't last long, because in 1939, with the construction of the nearby Wolf Creek Highway #47, the OR-8 designation was supplanted by a temporary routing of OR-2 until the highway was complete. Additionally, OR-6 was temporarily added to the OR-2 moniker in 1942 because its future routing was under construction. Essentially, adding routes OR-2 and OR-6 together equals OR-8, right? No? Okay, I tried. (That's what happens when I write these pages late at night.)
In 1949, with the completion of the Wolf Creek Highway (now the Sunset Highway), OR-2 is shifted there, leaving OR-6. When OR-6's highway, the Wilson River Highway, is completed 8 years later, OR-6 is removed from the Tualatin Valley Highway, and ODOT not only reinstates the OR-8 designation, but extends it along Gales Creek Rd. from Forest Grove to Gales Creek to meet up with OR-6. This new portion, however, is maintained by Washington County and not an ODOT highway. Oh well, can't win 'em all. The extension included a 0.23-mile co-designation with OR-47 in the heart of Forest Grove.
OR-8's current alignment from Forest Grove to Portland is basically the same as it was when it was first created in 1936. However, only the portion between US-26 in Sylvan and OR-47 outside Forest Grove is currently under state maintenance. A 1.35-mile portion of OR-8 that runs from OR-47 to the center of Forest Grove (called the Tualatin Valley Highway #29 Forest Grove Spur) was under state maintenance as of 2002, but was relinquished to Forest Grove maintenance around the same time the Nehalem Highway #102 (OR-47) was rerouted around Forest Grove (and thus eliminating the route duplex).