Boise and Oklahoma City are about 1,140 miles apart by road. Split the difference and you arrive near Denver, the city closest to the halfway point between them. That puts roughly 13 hr 21 min of driving on the Boise side and 10 hr 34 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. Denver still makes a fair, central place for Boise and Oklahoma City to converge, splitting the travel instead of asking one side to cross the country.
If Denver doesn't have what you're after, Salt Lake City and Albuquerque are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Boise and Oklahoma City.
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.