i fly oma-tpa probably once a month, and i usually have no problem finding K fares...out of OMA on friday evenings and back out of TPA on sunday afternoons. (and i get those great fares to boot)
i do find that it is difficult in the months of feb, mar, and apr to find K fares, but with some flexibility i usually have no problem getting a K fare one of the weekends i want.
it is a huge leisure market with limited availability -- definitely DL has a lot more availability (greater frequency of flights and larger planes).