Oki C5200: I've been recommended to clean the

Foreword: I suspect that what you really need to do is to check if one of the CMYK toner drums is dirty or scratched. Anyway...

When you open the printer completely, removing toner drums, to expose the paper path and fuser (see diagram on manual)

http://www.c5200.co.uk/index.asp?menumode=2&content=30005000fuser
http://www.c5200.co.uk/index.asp?menumode=2&content=30005000belt

...you will see a sort of "loom" made up of two to five thin metal wires. They're the "corona wires" and when operating they're kept at a voltage up to 20 thousand V (which is why you REALLY want to do this with the printer unconnected from the mains, and wearing rubber gloves and shoes just for luck). Their purpose is to neutralize the positive charge from the drum which makes the toner really stick.

With time, they may attract dust and grit which distorts the electric field shape and may result in "cloudy" images. Actually this is quite rare, because when the printer is turned off, the negative charge disappears and the dust simply falls down.

To clean the wires, use a dry cotton swab to very lightly and gently dust the wires themselves. If you see a black goo remaining on the swab, keep swabbing until wires are clean.

The rollers are the hard rubber rollers pushing the paper along. They too usually don't need cleaning (the big work is done by the belt, not by the rollers), and roller dirt is easily recognizable because the printed image has a pattern on it. If the pattern is less than one inch long, it's the rollers, whose outside circumference is small. If it's about three inches long, then you have a dirty drum or a dirty drum knife block (very likely, some dust accumulated there).

Rollers may be cleaned with rubber-safe cleaning fluid or isopropyl alcohol (in a pinch, common denatured alcohol will do). You can roll them gently with a gloved finger to clean the whole surface.

The drum (usually green) should be better left alone, but if you really need to, swab a cotton earwick along the border where it enters its plastic cover. Never touch the drum with your fingers: the skin oil will burn and completely ruin the drum itself. Same goes for any liquid, alcohol included. The best thing to clean a drum is dry compressed air from a can, kept upright to avoid freeze-jet. If the drum rolls without resistance, you can turn it one whole turn to inspect surface for sticking dirt or decolorations.

And since you're in the neighbourhood of the drums, check they haven't gotten vertical scratches - sometimes dirt will stick in the knife block instead of falling off, will get baked dry, hard and scratchy, and with time it will scratch grooves in the drum. This will result in vertical lines on the printed page - and the only solution to that will be a new drum unit.