Welcome to OFF. I grew up close to the Mary's River, and walked a good deal of it's banks as a kid. I can confirm that there are trout in it. That stream, and Greasy Creek that flows into it, south of Philomath, are good bets. At least they are the closest trout opportunities.
Park at the bridge by the Grange Hall, on Grange Hall Road, south of Philomath. From their you can work your way down Greasy Creek, a short way, to the Mary's. Then explore the Mary's from their. You can also work your way up Greasy Creek from that same bridge. It's going to be ultra low water, so "stealth mode" is the secret. But there are some 16-18" fish in there; but they spook easily.
There is also a pond out by Fort Hoskins that should have some cutties. But it's a bit sketchy so be on your guard. Packin' a side arm wouldn't hurt. 10-4?
Somewhat further away are the Alsea and Siletz Rivers. They will have some SRC's throughout each stream at this point in the seasons.
Good luck, tight lines, and let us know how you do.