An expressway would be any non-motorway road (or section of road) that has motorway characteristics, such as; 

a dual-carriageway, high-speed highway with a mix of grade separations and signalized intersections, driveways, etc. (like US 75 from Tulsa to Bartlesville);

a freeway that is undivided and/or low-speed (like the Duncan Bypass);

a two-lane highway designed to have a limited amount of intersections and a lack of driveway access (such as Gilcrease Expressway from its western terminus to where it becomes a motorway).

The access control, speed limits, and median all play a part in whether or not a road should be tagged as an expressway. It's a confusing, but useful tag.