Letâs start with the basics: Philomath , OR is located in Benton County . It has a population of 4,597, and we have a cool graph below that shows the cityâs ethnic diversity, as well as other important facts and figures. What about cost of living in Philomath , OR? The median income in Philomath , OR is $49,333 and the median home value is $194,600, which should give you a pretty good idea of its affordability. Philomath, OR City Profile Quick Facts