OR-15 was created at the inception of the Oregon State Route system in 1932, designated along all of the Pendleton-Cold Springs Highway #36 between, well, Pendleton and Cold Springs. This route was short-lived, as it became the routing for an extended US-395 in 1935; OR-15 was then decommissioned. US-395 was posted along this route until 1976, when it was realigned to I-82, OR-32, and I-80N (now I-84), creating the OR-37 designation along the route. I personally feel that they should bring back the OR-15 designation, as it would fit in better with Oregon's Primary State Route numbering scheme. Oh well.
The alignment of OR-15 is still completely intact. OR-15's route mostly lay along what is now designated OR-37 from Cold Springs Jct.; however, a roadway known as Cold Springs Rd. (shown in red) looks as if it were an original section of OR-15 between Holdman and Fulton. It looks like a new alignment of the highway was built from the Fulton area to the current junction of OR-37 and OR-334 at Kings Corners Junction (with the Athena-Holdman Highway #334, current OR-334, making up the rest of the new realignment between Kings Corners Junction and Holdman), so my guess is that OR-15 was only routed along the original stretch of the Pendleton-Cold Springs Highway and never along the new highway. When OR-15 was replaced, it was redesignated US-395 for 41 years, until US-395 was moved onto another highway to the west. US-395, during its tenure on former OR-15, was routed along the new highway, and was most likely routed along the older stretch until the newer section was built.