One building that I find interesting is the Beacon Building. Located on the Southwest corner of 4th and Boulder it was built by Waite Phillips in 1923. Many Tulsans remember it for a beacon light tower (removed in 1976) that symbolized its long-time tenant, the Beacon Life Insurance Company and served as a aircraft navigational aid in the 1930's. Link
Compare the two images below. In the first the beacon is visible in the background of the picture. In the second picture, taken in 2008, the beacon is no longer present.
The Beacon Building in 1935.
Another view of the Beacon Building with the Mayo Building in the background and the Adams Building to the side.
Black and white photos courtesy of the Beryl Ford Collection/Rotary Club of Tulsa, Tulsa City-County Library and the Tulsa Historical Society.
The Beacon Building Today