For all those owners of OKI B4100 printers and other lower models in B series, I composed a ppd file and attach it to this post.
I checked it works with Dapper and Breezy, might work with other distributions, doesn't work on FC5 (don't know why).
Basically, it's a heavily modified ppd for hp laserjet 6, and uses hpijs driver.
With the attached ppd you can use all OKI options, and you can set paper type, weight, 2 toner saver modes, tray/manual feed, etc.
With this ppd OKI prints at 600dpi. You can tell your printer to use 1200dpi too but hpijs doesn't support resolutions over 600dpi.
DON'T FORGET to switch off duplex printing in printer properties, otherwise each page you print will be followed by a blank one.
For those completely new to Ubuntu <like me >, download the attachment and double-click on it, then extract the .ppd file to a convenient location. Then go to System->Administration->Printing, Double-click on 'Add new printer', then follow instructions, when asked for printer model, click on 'install driver' and open your ppd file, then follow instructions again.