OP: Glad you are going to come out and hunt a unique animal in a cool place.
Lots of good advice above, but I’ll add my 2 cents b/c I grew up on the central Oregon coast: you are probably going to get wet no matter what. You can get wet because your breathable rain gear couldn’t withstand the wet brush, or you can get wet because you are doused in sweat from attempting to hike this steep country in ruberized bibs/slicker. Your choice.
Once you accept that reality, it’s more about dressing to stay warm while you are wet. Now, coastal Oregon in October isn’t really a “cold” place—I’d be surprised if it got below freezing your entire trip. But if you are wet and its 45 degrees F and blowing 20 (which will happen some or most of the time), you could be real uncomfortable (or worse) if you aren’t dressed appropriately. Your clothing doesn’t have to be sophisticated—just a base layer (merino is good, but polypro is fine too), a mid layer (fleece is my go-to), and something to cut the wind. Anything cotton or down is a no-go; you want material like wool or fleece that insulates when wet.
I don’t know the details of your hunt, but since there is basically no backcountry in NW Oregon, I’m guessing you are staying in a motel or lodge with a dryer, or at least a wall tent setup with a stove. If so, I’ll give some very un-Rokslide-y advice and say to prioritize quantity over quality in your clothing. That is, instead of investing in one set of amazing rain gear and under layers, I’d try to scrape together two or three sets of clothes. That way, you can keep drying stuff out as you go.
Good luck; hope you kill a monster blacktail.