logically it would be ko-opp if there were two words written like co-op.
as far as i know coop means (correct me if im wrong) co-operation, so it is two words, but now written in one. maybe its a logo/ci/cd-thing. for me its coop. the double-o gets stretched to a long sounding "oh"-> koop.
i remember that we had in german tv mid-80ies coop ads and i found a wiki-entry
http://de.wikipedia.org/wiki/Coop_eG plus an coop.de webpage. in those ads they were saying ko-opp. but its the same difference us germans saying Gel but hier its jeehl as the swiss tend to french-ize things. maybe this makes it coop even more...