Duplexers allow your Oki printer to automatically print on both sides of the paper. Many Oki monochrome and Oki color printer come with a duplexer installed as a standard option. If your Okidata printer model has a “d” in the model, i.e. C831dn, then your Oki printer was equipped with a duplexer as a standard option.
If your printer was not equipped with a duplexer as a standard option you may be able to install a duplexer as an option.