You may not have much luck with wildflowers and very little sun.

Pachysandra is my workhorse for deep shade here in the mid-Atlantic. It spreads great, it's evergreen, it doesn't mind having NO irrigation, it's self-limiting (as it fries in too much sun), and it doesn't try to climb and strangle everything.

The neighbors have vinca minor in deep shade. It's doing fine but isn't the same thick mat as the pachysandra. Ajuga does well with no irrigation. Hardy ferns are fine, but they don't spread all that well for me.

Hostas will grow that densely, too. So will hardy geranium--the native type is what I grew in NM without any irrigation in a natural low spot. It won't take super-deep shade.

In New Mexico, my go-to ground cover was English Ivy. It fried like an egg on a hot plate in the sun, and the near-desert conditions meant I could keep it in bounds. Not in your zone, though. It'd eat everything. (In NM, I'd go after anything that was invasive in other states but not where I lived. That meant it'd likely be a survivor but not dangerously aggressive!)

I had some invasive wild rose that would grow in shade there, too, and some perennial sweet pea there.

Give up on the wildflowers. They won't do in deep shade.

I use Periwinkle (vinca minor) under our deep shade deck and it is really wonderful. Really spreads fast and is still flowering in October in zone 4.

Where in Oregon are you located? What zone? If in a mild area (zone 7 or 8), you have a lot choices :-) The pachysandra mentioned will work, also the vinca. Euphorbia robbiae is a wonderful evergreen groundcover for a tough, shady area, as is Sarcococca humilis, which also offers the bonus of fantastically scented winter flowers (common name sweet box). Both can tolerate very dry conditions if necessary. You could also choose one of the natives that works well in this situation, salal (Gaultheria shallon) or Oregon grape. Native ferns - western sword fern, deer fern - will also thrive in this location.