(Borrowed from South Ridge Route; same starting point. Except for winter, you may have to settle for parking where the snow gets bad and skiing the rest).
From the west take I-5 to Highway 58 in Goshen, OR. (From the east you'd have to take Highway 97 to Highway 58) From Goshen, take Highway 58 35 miles east to the town of Oakridge. Just outside the the east edge of Oakridge, you'll take a right onto Road 23, only problem is that it's not marked so you have to look for a large green sign that says "Kitson Spgs Rd/Hills Creek Dam/Middle Fork Willamette". Turn right at that sign and go .5 miles to Road 21 (also not marked - look for the sign that says "Diamond Drive"). Turn right onto Road 21 (also known as Rigdon Road) and go 30.6 miles to Road 2154. Take a left on 2154 and go .3 miles to Road 380 (the sign has a large 380 on it with a smaller 2154 as well). Turn left on Road 380 and go 3.7 miles to Road 2160. Turn left onto Road 2160 and go 1.6 miles to the trailhead. There is a brown hiker sign and the trail is marked Trail 3632.
Approach the mountain by climbing from trailhead up to an upper plateau through trees --- you won't be able to see the mountain until you crest the plateau, so do your orienteering to stay on track. Once you get on the high plateau, on level with the base of the mountain, the trees thin out and you can see a prominent NW ridge. Angle left and climb onto the ridge, then follow it eastward, ascending. Near the summit, the ridge merges with some other (N,E) ridges and heads south to the summit.
Although most of the route is a cake-walk, there are several steep sections in the middle of the ridge where you need to traverse around gendarme-like blocks. If the snowpack is hard, you will want crampons.
I have regularly seen the coolest cauliflower rime formations on top of Diamond Peak. Very fun...
If you've had the gumption to carry skis up, it's fun to bail off and ski the front bowl/face.
For winter climbs:
Skis or snoeshows are a must to get in during winter, unless there snowpack is really consolidated.
Up higher, an ice axe provides security; crampons are mostly optional, though they can help on the one or two steep/exposed parts on the ridge ---although I believe all of these could be avoided if you are willing to traverse WAY around.
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