Marion Reservoir has four areas that have been developed for camping and recreational use. These are Marion Cove, Hillsboro Cove, Cottonwood Point, and French Creek Cove. They provide a mixture of amenities including showers, grills, electric hookups, dump stations, boat ramps, beaches, etc. Though this is a popular camping spot during the summer months there are usually plenty of spots available.
To check conditions of the boating ramps and other possible information needed, check out the USACE Tulsa District web page.