On January 21, 1867, the White River Post Office is established along the river of the same name (later renamed Green River). The river is the main transportation route to Seattle, and the post office sits adjacent to the ferry dock. Lewis McMillin is appointed first postmaster. He is paid $1.71 for the first six months of operating the post office.
The White River Post Office was located on the east side of the river within the present city limits of Kent between S 216th and 224th streets. Kent is located in King County about 15 miles southeast of Seattle.
The White River Post Office took the place of the D'wamish post office, which was located nearby and closed in 1864. In 1889 the town had a Baptist and Catholic church, a school, saw mills, a blacksmith shop, a hotel, a shoemaker, a physician, and a general store. It was at the center of a large hop-growing region. (Hops are the main ingredient of beer.)
The post office did not operate between October 23, 1868 and December 27, 1869.
On February 6, 1890, the post office changed its name from White River to O'Brien, the present name of the area. At the time of the name change, the post office moved two miles east to the Northern Pacific Railroad line, which was now the main transportation route. Christian Thygesen was the postmaster at the time, and operated the post office out of his general store.