Ok no longer flies to the USA.

What happens with these partnerships is that a certain number of award seats are available on partner flights. For example, you probably can use OK miles to fly with DL, but maybe OK is allotted only 6 award seats on the flight and those seats may already be claimed by other people. There still may be plenty of empty seats on the plane, just none designated to OK.

If your dates are flexible, you should be able to find something. Keep trying.