Beginning at O’Leary / Castle Rock Trailhead, O’Leary Mountain Trail #3321 heads east, switching back steeply up a large ridge through an old growth conifer forest. The trail continues uphill, following Forest Road 3321 until it passes below a panaroramic viewpoint on Macduff Mountain. Passing the mountain, the trail drops into a saddle, then sidehills east below McLennan Mountain. The trail occasionally breaks out of forest into rock outcroppings, then hooks north to reach its terminus at Olallie Trail #3529.
The steep switchbacks near the West end of the trail are being re-routed currently.
Trail description from the US Forest Service website.
Most people ride this trail as part of a clockwise loop with Olallie, O’Leary, Castle Rock and King Castle. That way you can climb a significant amount of road which will get you pretty high up on the loop before decending 100% singletrack. The full loop is over 30 miles and 6000+ ft elevation — so brace for a long day.