Oklahoma City and San Antonio are about 421 miles apart by road. Split the difference and you arrive near Dallas, the city closest to the halfway point between them. That puts roughly 4 hr of driving on the Oklahoma City side and 5 hr 18 min on the San Antonio side — the fairest single meeting point among the cities near the middle.
That's a half-day drive from each side, so Dallas suits an overnight or a weekend rather than a quick coffee — long enough to want a reason to stay, short enough to drive.
If Dallas doesn't have what you're after, Austin and Houston are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Oklahoma City and San Antonio.
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.