I have the Oki 5650 which had been great for about 18 months. Then found that the C cartridge was showing error as 'not installed'. After much fiddling about with it and checking on the internet I can now rectify this by either pushing down on the top of the printer while it boots up, or removing the small chip on the left top of the toner (make sure you have 'earthed' yourself to prevent any static damaging the chip when you handle it!) and taping securly onto the top of the cartridge above the chips proper slot. This now has been working OK for 6 months. Strange how this only affects the C cartridge (which has also been replaced and had the same problem). I suspect that the printers RFID chip reader for the C cartridge is having problems reading the chip and moving the chip closer to the reader by either taping to the cartridge top of puching on the top cover while printer boots up resolves the problem (for now anyway!) This can also trigger the 'service call 231 fatal error 03' message which also disappears if you follow the above procedure.
Hope this helps you.