This broken-down structure is in the E. E. Wilson Wildlife Area. The wildlife area occupies part of a larger area that was used in World War II as a military complex called Camp Adair. I don't know what this particular structure is, but it's clearly in a state of "ruin". As you look around, notice the various monuments. Each is a dedication to some unit that was involved with the camp during the war. Once a small town, the area is now returning to its wild state, broken occasionally by the remnants of structures such as this.
This location is just about in the center of the area encompassed by the camp. As an active wildlife area today, it is used by hunters, photographers, wildlife observers, and anyone else who likes to explore the roads and trails. Motorized vehicles are not allowed, but bicycles are a great way to explore the further reaches. There are a number of ponds, streams, and marshes that are full of wildlife. There is fishing, observation platforms, an interpretive trail, and even an archery park.
The wildlife area is near the town of Adair Village, and is managed by the Oregon Department of Fish and Wildlife. If you look to the right of the structure a bit, on a line with the straight stretch of path, the location of my 2010 project for "Forgotten Places" is about a third of a mile away (about 600 meters).