There is a page counting chip on the drum unit. When the drum gets to a certain number of prints the printer shuts down the chip. The printer counts four passes or four pages for each colour print. If your drum was working correctly prior to the "replace drum" appearing, you can replace the chip on your drum unit and continue printing.

One chip required per drum.

The chip will not fix any print defects or quality problems it simply allows the drum to continue printing.

http://www.zinetic.co.uk/oki_c9655_drum_reset_chip-details.aspx