Bruce,
I can tell you that it's flat. Of course, that is most of Oklahoma (my Dad lives in Oklahoma City).
Not exactly correct. Maybe a third of Oklahoma could be considered "Flat". From OKC to the west, but the Eastern half of the Sooner state has small mountains and large pine trees. It is home to the only mountain range in the U.S. that runs East to West, the Ouachitas which were once as tall as the Rockies. Lumber is big in far southeast Oklahoma. They may not be the Rockies but Oklahoma is home to 7 minor mountain ranges from the Southwest, Southeast and Northeast. They are Arbuckle, Glass, Kiamichi, Boston, Ouachita, Ozarks, and Wichita.
There are also lots of hills. I live north of Tulsa amoung the Osage Hills, there are the Antelope Hills in Roger Mills County, Oklahoma. They are a series of low hills in the bend of the Canadian River. The highest peak has an elevation of 2585 feet. They were a major landmark for the Plains Indians and travelers on what is now the western plains of Oklahoma. The hills are on the National Register of Historic Places. The Red Hills are a grouping of large red colored hills in south Kansas and north west Oklahoma. They are also referred to as the Gypsum or "Gyp" Hills, especially in Oklahoma, after the mineral capping this series of erosional mesas and escarpments. They are part of the Permian Redbeds region in Oklahoma and the Red Hills Prairie region in Kansas. The Red Hills have large vistas and some small steep canyons. The highest elevations reached in the range are on Twin Hills (2,708 feet (825 m)) and Table Mesa (2,643 feet (806 m). The world's highest hill, Cavanal (elevation 1,999 ft. above sea level) is located just west of Poteau in LeFlore County.
With 200 man-made lakes, Oklahoma has more man-made lakes than any other state and boasts over one million surface-acres of water and 2,000 more miles (3,200 km) of shoreline than the Atlantic and Gulf coasts combined. Lake Eufaula is the largest lake in the state, covering 102,000 surface acres. I mention the lakes because almost everyone of our man made lakes are surrounded by hills and small mountains.
Oklahoma has some salt flats that start just west of Ponca City and Enid and stretch through the panhandle but don't get the impression that most of Oklahoma is anything like Kansas, which is definetly flat.