Garthy: You are in a better position than most of us to answer this. If your Oki is not very old and a single pass colour printer, why not have a go on an inside back cover from one of the many company reports and other sources of quality paper? There's an infra-red heater to melt the toner on exit from your Oki, so plastic coated surfaces are better avoided.
More than one review suggests laser/LED printers are less fussy than inkjets about paper surface.
I intended to get an Oki for photo prints, but was diverted by the '50 for £5' Boots offer. They print with a laser and I asked if I could buy a sample of the photo paper they use. The saleslady said no, but with a smile she found several off-cuts for me to try out. A boost for customer relations!