Oki C 5400n - Manual (Page 28)

Menu Functions • 26
6. Press MENU + to change the setting to ON:
DUPLEX
ON
The second line of the display begins flashing.
7. Press ENTER to select the new setting:
DUPLEX
ON *
An asterisk appears on the second line of the display.
8. Press ON LINE to engage the new setting and to exit the menu.
The printer returns to standby mode.