Oki C3520 - Manual (Page 93)

Maintenance > 93
3. Press the cover release (a) and open the top cover of the
printer (b) fully.
4. Note the positions of the 4 cartridges/toner cartridges.
It is essential that they go back in the same order
a
b
1
2
3
4
1. Cyan cartridge 2. Magenta cartridge
3. Yellow cartridge 4. Black cartridge