This should work for C3XXX, C5XXX and C7XXX series.

I never wanted to do the NVRAM complete reset because I was scared of losing the network MAC address as mentioned at some forums. Controlling the consumable level independently was much more interesting. Here is the step by step instruction:

Get to the standard “Diagnostic mode”:

1. Turn off printer.
2. Turn on printer, while holding the + and ? buttons.
3. The LCD Shows “OKI User”, Scroll to “Engine Diag Mode”, press “enter”.

The display shows “Diag Mode 01.02.02 S-Mode” as mention multiple times on this thread.

At this point you should wait for the initialization cycle to complete. I mean for all the motors and gears to have finished their noises and cycles ? otherwise you will end up with a LCD switching between two different info screens – not a big deal but still annoying.

Now we want to go in “regist mode” by:

1. Pressing the “Online”, “Cancel”, “Enter”, “-” and “Back” AT THE SAME TIME (not in any sequence ? at the same time). Yes! That is 5 buttons.
2. After within two seconds the LCD menu will show “READY”. If it doesn?t, release button and repress buttons again.
3. The buttons release sequence is important. So, read this before letting go your five fingers: Release the “Enter” button first and THEN the four others (for some firmware ?On Line? must be released first). LCD is still showing “READY”.
4. Press the “Online” and “Cancel” button at the same time BRIEFLY. Don?t wait before pressing them after the five fingers sequence otherwise you will miss your time slot.

You can always switch back from the “READY” to the “Diag Mode 01.02.02 S-Mode” menu by pressing the five buttons again.

Don?t laugh. I know it seems a lot of gymnastic. Well I guess they didn?t want the user to find that one easily.

Now you see “Eng Diag Level 2″

Press either the + or ? button until “CONSUMABLE COUNTER SET” is displayed, press the Enter button; you are now in the menu to set various consumables.

1. Press either the + or ? buttons to scroll through the various consumables, at the desired consumable (ie. “K-ID UNIT ?” for the Black Image Drum Unit, “C-Toner” for cyan toner, etc.), press the Enter button and the last digit of the count will start flashing.
2. Press the Cancel button to decrease the digit amount or press the On-line button to increase the digit amount; press the + or ? buttons to move along individual digits in the count. Note that it will not show the remaining but the usage (100% means it is empty).
3. Once the count is at the level you desire, press and hold the On-line, Cancel simultaneously until the numbers stop flashing; you have reset that consumable?s count to the number you specified. Note that you may end up with a different value then the one that was entered when saving. The printer engine is probably jumping to the nearest registered/acceptable value. If someone find a way to save the exact value, let us know.
4. Press the + or ? buttons to move to the next consumable and repeat at step 1
5. Once all consumable counts have been set to desired amounts that are below their end-of-life thresholds, you can use the Back, +, & – buttons to exit through the menus until “INITIALIZING” is displayed. Or turn printer off and on.

You’re finished! Now log in to the web interface with your browser and check to see the remaining life of your consumables.

Good luck.