Oki C5550 MFP User Manual page 192

3. If using the PS driver, on the [Color] tab, click
[Grayscale], and then click [Advanced] if you want to
adjust brightness and contrast.
PS Greyscale_oda.jpg
Some applications will attempt to set the halftone screen
information for the MFP. If the option to [use printer
halftones] is left at ON, the MFP will set its own halftone
screen value, giving a smoother result. It is recommended
to leave this option on.
[Optimized grayscale halftones] may take slightly longer to
print but will give best quality.
C5550 MFP User's Guide
A guide to printing > 192