The FAA chart user's guide says:
Hard-surfaced runways greater than 8069' or some multiple runways less than 8069'
It's the latter case of some multiple runways less than 8069'.
This is also confirmed in the post, "https://aviation.stackexchange.com/q/33905/14897"
Specialists also place these polygons around the runway pattern of aerodromes with multiple runways that are less than 8069 feet, in cases where the multiple runway pattern does not fit into the largest, circular aerodrome symbol.
(FAA FAQ; emphasis mine.)
Most likely MKO doesn't fit in the standard circle because of 36-18 and/or 4-22 intersecting the circumference.
(Chart Supplement via skyvector.com)