I farmed in Southern Oregon for 10 years and grew lots and lots of onions. So though I certainly don't know everything, I do have some info to offer.
For starters, onions seeded indoors in late winter, then transplanted out and given good fertility, can grow large and well. Most long day varieties will do just fine. Anyone that told you spring seeded onions won't do well there doesn't know what they are talking about. In fact, those sunny, dry March weeks are the best time to transplant onions (I wish we had more of them here in the Willamette).

Overwintering onions are a different story. You can't grow most long day onions over the winter or they'll bolt. Walla Walla is one great option for sweet onions (or Alan Venet's Siskiyou Sweet, same thing). For yellow onions try Top Keeper, Hi ball, T40 (all Japanese hybrids). These are all bred to overwinter and bulb in early summer. None of the overwintering onions will keep much past November though, you need to grow some spring seeded keepers for that.

I can go on and on but here is my advice in a nutshell:
Choose the right variety for the season, experiment for the best timing, always provide enough fertility and water.

Hope this helps.

-Mark

Definitely good stuff to work with! Thank you so much for the suggested types too. I never heard of most of those, so that was very helpful. Last year was my second garden and I am amazed at how much there is to learn about gardening. I love learning about it.