Las Vegas and Oklahoma City are about 987 miles apart by road. Split the difference and you arrive near Albuquerque, the city closest to the halfway point between them. That puts roughly 10 hr 7 min of driving on the Las Vegas side and 10 hr 49 min on the Oklahoma City side — the fairest single meeting point among the cities near the middle.
Over this distance most people will fly rather than drive the whole way. Albuquerque still makes a fair, central place for Las Vegas and Oklahoma City to converge, splitting the travel instead of asking one side to cross the country.
If Albuquerque doesn't have what you're after, El Paso and Denver are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Las Vegas and Oklahoma City.
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.