Where is Walnut City? Actually - Walnut City is a moniker, not a place in Oregon!
According to the Oregon Travel Information Council:
Andrew J. Baker, who arrived in the Oregon Territory in 1843 as part of the Great Migration, planted the black walnut in 1870 next to his house, built in 1852. The house, which still stands at this site, was used as a stage coach stop in the late 1800s. The property was purchased in 1899 by William S. Russell and is still owned by Russell's heirs.
The tree is located on private property seven miles west of McMinnville at 17676 SW Oldsville Road. It can be viewed from Oldsville Road.