Oklahoma City and Omaha are about 409 miles apart by road. Split the difference and you arrive near Kansas City, the city closest to the halfway point between them. That puts roughly 6 hr 15 min of driving on the Oklahoma City side and 3 hr 28 min on the Omaha side — the fairest single meeting point among the cities near the middle.
That's a half-day drive from each side, so Kansas City suits an overnight or a weekend rather than a quick coffee — long enough to want a reason to stay, short enough to drive.
If Kansas City doesn't have what you're after, Dallas and Memphis are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Oklahoma City and Omaha.
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.