"Over the Crooked River Highway from its junction with the Ochoco Highway, US26, in Prineville, southerly via the Crooked River and Bear Creek Canyons to its junction with the Central Oregon Highway, US20, approximately seven miles west of Brothers."
~ ODOT, Descriptions of US and Oregon Routes, March 2007
OR-27 was created at the beginning of the Oregon state route system in 1932. Its original routing began on Warm Springs Highway #360 at Madras at the junction of old US-97 (then a portion of The Dalles-California Highway #4, now OR-361, Culver Highway #361) and continued across many county roads (roughly along US-26's current path) through Prineville south along the Prineville-Bear Creek Highway #14 (roughly along OR-27's current path) south to US-20 near Brothers. At the time, the Warm Springs Highway #360 ran northwest of Madras to the Jefferson/Wasco County Line, but this section was not defined as OR-27. Neither was a further northwestern extension to the Wapinitia Highway #44 on September 28, 1938.
There is some confusion surrounding how OR-27 was routed in the field prior to about 1940, as well as when the northern terminus was moved to Metolius and back. OR-27 was defined in both 1932 and 1934 as "[b]eginning at a junct. with U.S. 97 at Madras and following Secondary #360" to Prineville. The Jefferson County portion of Warm Springs Highway #360 was defined in 1933 as "Market Road No. 9, beginning at the Crook-Jefferson County line and extending through Lamonta to Market Road No. 8 and thence down Willow Creek to Madras". The absense of any mention of Market Road 3, which ran from Metolius through Lamonta to Grizzly, implies that the highway (and by extension OR-27) was intended to take a new routing between Lamonta and some point along Market Road 8 about 5 miles due north. Some commercial maps (see 1935 Gousha map segment below) do show a dirt road along that corridor, but it is absent from official Oregon maps. In addition, while OR-27 was shown on official maps between Prineville and Brothers starting in 1933 when all other posted state routes first appeared, it didn't appear north of Prineville until the 1937 edition. I would imagine that 1937 was also around the time the northern terminus moved to Metolius by rerouting the highway along the western part of Market Road 3. This routing was shown on commercial maps sometime between 1938 and 1940 (see 1941 Gousha map segment below). The 1943 route description verifies that OR-27 starts at US-97 "approximately 1 mile north of Metolius".
On July 19, 1940, the portion of the Warm Springs Highway #360 northwest of Madras was reclassified as a primary state highway, Warm Springs Highway #53; the remaining portion of the Warm Springs Highway #360 (essentially all of OR-27) was redesignated as the Madras-Prineville Highway #360. After WWII, new sections of US-97 and OR-27 were constructed, moving the northern terminus to just south of Madras. However, this was short-lived; the Madras-Prineville section of OR-27 was deleted when US-26 was extended into Oregon in October 1951. Additionally, in 1961, the Prineville Reservoir and Bowman Dam were built, requiring an entirely new highway to be built around the reservoir. Once completed, the highway was redesignated as the Crooked River Highway #14, the name it has today. Since then, it's pretty much remained the same old mostly-unpaved road for the past 60+ years. Personally, I think it's a useless designation south of the reservoir, but if you want to take the road less traveled, OR-27 is the way to go.
In 2015, ODOT identified OR-27 as a possible route to turn back over to Crook and Deschutes Counties. As part of the deal to relinquish OR-27 and two other highways (OR-370 and OR-380), ODOT would take over jurisdiction of George Millican Road, also known as Les Schwab Highway. This county road runs a couple miles west of current OR-27 between roughly the same two places, but is paved and straighter. Seeing as how the two roads get confused with each other a lot currently (Wikipedia says that the unpaved OR-27 is the "Les Schwab Highway", and news stories commonly say that the paved Millican Road/Les Schwab Highway is OR-27), such a swap might be a good idea. It is unknown at this time what number, if any, Millican Road will receive in such a swap, but OR-27 makes the most sense to me.
In 1935, OR-27 ran from US-97 in Madras to OR-54 (future US-20) near Brothers. This map only shows it from Prineville to OR-54 though. Note that none of the routing is paved. (©1935 Gousha)
Gousha showed the routing of OR-27 to Metolius on their maps by 1940. (©1941 Gousha)
OR-27 is moved onto a new section of highway coinciding with the opening of a new alignment of US-97 in 1948, moving the northern terminus back to just south of Madras. The entire routing between Madras and Prineville is now paved. (©1948 Gousha)
OR-27's routing is shortened to its current termini in 1951 with the extension of US-26 into Oregon, as evidenced in this 1958 map. Note the pre-resevoir routing. (©1958 Gousha)
A portion of OR-27 was put underwater in 1961, requiring a new routing near Taylor Butte as shown in this 1965 map. The majority of the highway is still not paved. (©1965 Gousha)
This is the routing of OR-27 as of 1977. It looks as if the Prineville Resevoir receeded a little bit, enough to create a state park along its old alignment. It also could be because maps still were not computerized at the time. (1977 State of Oregon)
This is the current routing of OR-27 as of 2005. The route is now paved all the way to the crossing of Bear Creek, but still leaves 18½ miles unpaved on its southern end. I hope it never gets paved. (©2005 AAA)
It is important to note that all map segments are copyrighted by their respective owners, and that these map segments are used for educational and historical purposes only.
A portion of old OR-27 exists between Metolius and Lamonta, which disappeared in the late 1940's. Through route reconstruction, I believe the highway, shown in red, left Metolius eastbound along 6th St., then turned south along SW Columbia Dr., then turned into SW Eureka Ln., then followed SW Bear Ln. crossing the current right-of-way for US-97 (a small section is missing where they had to make the road intersect US-97 at proper angles). After that, the road's actual route gets a little hazy.