| Obstruction Peak to Deer Park | Olympic National Park | |
| This trail leads seven and a half miles one way along Grand Ridge. The ridge forms a divide between Morse Creek and Cameron Creek drainages in the northeastern Olympic Mountains. |
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| Oklahoma Trail | | |
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| Old Ellensburg Trail | | |
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| Old Gib Trail | Wenatchee National Forest | 9.5 |
| Old Geo. is a seldom maintained but scenic old trail along a ridge from Estes Butte to Cairne Mountain. |
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| Old Government Trail | Mt. Baker-Snoqualmie National Forest | 2 |
| The Old Sauk Trail is an easy 3 mile hike. The trail begins 0.7 mile south of Clear Creek Campground on the Mt. Loop Hwy. and ends at Murphy Creek. |
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| Old Pipeline Bed Trail | Wenatchee National Forest | |
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| Old Sauk Trail | Mt. Baker-Snoqualmie National Forest | 3 |
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| Old Stage Trail | Colville National Forest | 5.8 |
| Due to its historical significance there are various interpretive steel and rock cairns along the route. There is water available from small streams as well as stocked water from a pine wood trough at the historic Lust Spring. |
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| Oldgrowth Loop Trail | Gifford Pinchot National Forest | 1 |
| This loop trail takes off from Woods Creek Trail #247. It is barrierfree, but is more difficult than the main loop. The trail is 3 feet wide and is a mix of native surfacing and compacted gravel. Grades increase to short sections of up to 20 percent. F... |
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| Oldman Trail | Gifford Pinchot National Forest | 12.4 |
| This section of the trail is also groomed and ski tracks set. The Scenic Loop Trail is best skied in a counterclockwise direction to avoid steep grades at the loops' north end. Trail #148 from Oldman Pass Sno-Park to Road 3050 Sno-Park is an easy one w... |
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| Olympic Hot Springs | Olympic National Park | |
| This trail leads along the now closed Boulder Creek portion of Elwha River Road to the parking area two miles beyond the closure. The spur trail leads .5 miles southward from the parking area. |
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| Osborne Mountain Trail | Gifford Pinchot National Forest | 5 |
| Trail #250 begins in Big Creek Campground (Road 5200.029) and climbs steeply through a moderately dense forest. The trail passes through a harvest area and climbs to the south side of Osborne Mountain. The trail is cut by a switchback from Road 8410 (a... |
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| Osborne Mountain Trail | | |
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| Oso Truck Trail | | |
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| Oval Pass Trail | Wenatchee National Forest | 1 |
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| Owl Creek Spur Trail | | |
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| Owyhigh Lakes | Mount Rainier National Park | |
| This trail leads one way from White River Road to State Road 123. It provides access to a primitive camp, alpine lake and flower-filled meadows. |
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| PJ Lake | Olympic National Park | |
| This trail descends steeply for one mile to PJ Lake, which lies north of Hurricane Ridge at the head of the Morse Creek drainage. |
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| Pacific Crest | Okanogan National Forest | 70.2 |
| This popular trail is the final leg of the Mexico to Canada trek. This is also the major north-south trunk trail of the Western Pasayten with many east-west branches, making many loop trips possible. |
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| Pacific Crest National Scenic Trail | Gifford Pinchot National Forest | 50.4 |
| The Pacific Crest National Scenic Trail is 50.4 miles long and located in the Mt. Adams Ranger District. |
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| Pacific Crest Stevens Pass to Stehekin River Road | Wenatchee National Forest | 98 |
| This 98-mile stretch of trail features some of the wildest, craggiest terrain on the 2,600-mile-long route. But you have to work for the views. To avoid avalanche chutes, glaciers, and permanent snowfields, the trail descends into valleys and climbs ba... |
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| Pacific Crest Trail (Kodak Peak to Suiattle Pass) | Mt. Baker-Snoqualmie National Forest | 49.6 |
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| Pacific Crest Trail (North) | Mt. Baker-Snoqualmie National Forest | 31 |
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| Pacific Crest Trail (Part I) | Mt. Baker-Snoqualmie National Forest | 10.2 |
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| Pacific Crest Trail (Part II) | Mt. Baker-Snoqualmie National Forest | 12.9 |
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