Only fields I know around Corvallis are Diamond Hill in Harrisburg, the two fields in Lebanon (know almost nothing about them), Albany closed down a few years I believe, then there's Warpaint up in Salem. There's also Paintball Palace in Eugene, but that's in Duck land.
Diamond Hill is open every weekend I believe, and is one of the host fields for the WCPS tourny series.
Don't know anything coming up at Warpaint itself, but they are sponsoring Black Hawk Down at Sniperz Den in St. Paul on February 23rd.
I'm thinking of going to Black Hawk Down in February, it's easily been over a year since I last went paintballing.