Two passes to choose from
a) McKenzie Pass on Hwy 242
Seasonally open June to October, sometimes earlier for cyclists
Much less traffic, much more scenic, almost 1000 feet higher
Narrow road, mostly without shoulders, but traffic is not in a hurry
b) Santiam Pass on US 20 / 126
Open year-round, main connection between western & eastern Oregon
Moderately heavy traffic, fast-moving, easier grades and lower elev.
Fairly good shoulders most of the way, but nothing special to ride
If you have time westbound, you can take Hwy 242, then swing east (actually north) on Hwy 126.
Both of these sections have fairly low traffic and you get the best of both worlds.
West of the Cascade passes -
Hwy 126 - The ACA route has good services and light to moderate traffic, mixed shoulders.
US 20 has the least traffic, but the most climbing, lots of up & down, few shoulders, limited services.
Hwy 22 has moderate traffic which increases after Detroit Lake, moderate to wide shoulders as you head west.
Via US 20 - west of Sweet Home you can get on county roads.
Via Hwy 22 - west of Lyons you can get on county roads.
If you are heading to the coast, US 20 then back roads thru Corvallis to Newport is quite nice.
If you need to make time, Hwy 22 is o.k., try to catch the Wheatland ferry north of Salem.