Upgraded to Mojave made OKI C531dn printer print images blocky. Solved!

Go to OKI support page https://www.oki.com/uk/printing/support/drivers-and-utilities/colour/01327801/ & download both the updated PS printer driver AND the Driver PS Updater and install.

Then open Printers & Scanners in System Preferences and delete the OKI C531dn from the list of printers. Close Printers & Scanners, reopen and add the C531dn back into the printers list. You will see the "Kind:" named as, OKI C531(PS). Set the printer as the Default Printer and close Printers & Scanners.

you should now be able to print photos and images. I could!