Oki ES6405 User Manual page 71

Starting with the cyan image drum unit nearest the fuser,
8.
replace the four image drums into the drum cavity, making
sure to locate them in the correct order.
Holding the complete assembly by its top center, lower it into
place in the printer, locating the pegs at each end into their
slots in the sides of the printer cavity.
71
– ES6405 User's Guide