GOOD presents information on circulation v. area population of the 75 largest newspapers in a cool, visual way. The data is from 2006, but it's still interesting.
Note: The Oregonian says pop. 550,000; daily circulation 333,000. It seems like The Oregonian has a strong hold on Portland, but I am guessing the population is JUST for Portland, whereas the circulation includes tons of people who live in Lake Oswego, Greshman, etc. So, the real level of penetration may be quite different.