Moon Mountain lies on an east/west ridge in the Aldrich Mountains in central Oregon between the John Day River and Canyon Creek. It actually sits apart from the rest of the peaks on this ridge on a smaller ridge that extends north from the central point on the ridge. From east to west on this ridge are Fields Peak, Moore Mountain, Moon Mountain, McClellan Mountain and Riley Mountain. A trail starts from either Fields Peak or McClellan Mountain and makes for a nice long day of peakbagging while walking the ridge.
The Aldrich Mountains are a subset of the Blue Mountains, a huge range of peaks that extend from Smith Rock near Bend, OR east and then northeast towards southeastern Washington. The rock is volcanic (and therefore mostly crappy) but the mountains were caused by uplift.
The trail that runs the ridge must be departed from once you reach the smaller ridge in between Moore Mountain and McClellan Mountain that extends north to Moon Mountain. From there you go "cross-country" although it's mostly above treeline and covered with small scrub brush and flowers. Great views from the Ochocos in the west to Strawberry Mountain in the east. Elevation gain would be roughly 2900 feet cumulative from the west trail and about 5 miles from the trailhead.
Take Highway 26 fron the west (Portland) or the east (Idaho) to a point about 13 miles east of Dayville, OR. Turn south on Fields Creek Road where there is a sign for Murderer's Creek Guard Station. Follow this 8.2 miles to a sign for Fields Peak and Road 115. Turn left onto this road and go about a half mile to a "T". Turn right onto Road 2160 and go 200 yards to Road 041 and turn left. Take this rough road about a mile to its end and the trailhead.