The good fellow above me speaks the truth.
I'm in the Monmouth area, but I attended highschool in Corvallis. There are NO officially Airsoft fields in the Corvallis/Albancy/Monmouth area. The closest you'd have to go would probably be a little outside of Salem for anything official.
Unofficially, there are a lot of 'backyard Airsofters' in Corvallis that still Airsoft every once in awhile. I happen to know some of these guys, and we managed to squeeze a quick, nice game in a relatively well sized forested area over the Thanksgiving break.
Though, if you want to the good, real Airsoft stuff, go to (for example) Blackwater Tactical in Salem to get more information. And keep your eyes peeled on the forums and stuff.
I'd tell you to be careful. It's not the Tank you need to fear.It's Slumberman