An amazing single track descent makes this 5 mile climb and 2,000 foot elevation gain on a forest road well worth it! You will actually descent the ridge of O'Leary Mtn even though the name of the trail is Olallie Mtn which is several miles away.
From I-5 take state Highway 126 from Eugene until you get to the town of McKenzie Bridge. Turn right on Forest Service Road 2683 "Horse Creek Road" just past the town. At 1.8 miles down the road take a right onto Forest Service Road 1993. Continue up this road for almost 3 miles until you see a parking pullout on the left. Park here and start riding up the road.
Start your 5 mile ride up Forrest Road 1993. Sometimes the road is steep and you'll definitely be tired, but trust me, the descent is well worth the effort. If you have two cars you could easily make this a shuttle run taking turns on the single track descent!
From the top of this final ridge you should be able to see Mt. Washington and the Three Sisters. If it is spring time you'll probably see a bunch of wildflowers too. From here keep an eye out for Forrest Service Road 340 which you'll spot behind a big green gate. At first the loose gravel road climbs steeply but then it levels off and even descends a little bit before traversing to a clearling and road junction.
A sweet panorama of the view from the top of the Olallie Mountain trail can be had here. On clear days you should be able to see all the way to the Sisters and possibly even to Jefferson and Mt. Hood in the far distance. In the summer you should see plenty of wildflowers as well. Start heading down the left fork in the road but GO SLOWLY or you'll miss the single track tail heading into the woods on your right... it's only about 0.1 miles from this point.
Olallie Trail 3529 is a single track mountain bike trail that can be easily missed if you don't go slowly enough. It doubles back down the ridge from which you are riding away from so be careful not to pass it or you'll be biking back up the hill to find it! You'll now have 3.5 miles of amazingly smooth single track to rally down. Enjoy!
Warning - Some switch backs near the end before it dumps you out at the parking lot again. Be careful not to be going to fast or you may fly off the trail!
The video below isn't the whole descent but it's a a good seven minutes of it. It'll give you a good idea of what you are in for. :)