I’m not holding my breath about world peace, and I’m not any more sanguine about consensus on the prefix co-, but here’s an appeal about coming to terms about this term.
Generally, no hyphen is required to link this prefix to root words. In the case of many prefixes, the element is initially attached to the root word, but as readers and writers become accustomed to the new construction, the linking device is omitted, and the components are joined.
However, some constructions resist this transformation because the resulting fusion looks odd. We make exceptions for certain contiguous vowels (anti-inflammatory) or for some awkward-looking combinations (pro-choice, though proactive and most other pro- constructions are closed). But the default setting, at least in American English, is to discard the hyphen and close up the resulting space.
Among the difficulties (for careful writers and most editors, at least) is that inconsistency is jarring. For example, codependent and cohabit are closed, but co-occurrence and co-owner, because of the proximity of the two os — which would, presumably, collide like bumper cars if not for the restraining influence of the hyphen, despite the fact that the components of cooperate manage to, well, cooperate just fine — are hyphenated. What do you do when one or more of that first pair of words appears in proximity (or even in the same article or book) to one or more of the second pair?
One solution is to get over it and allow the discrepancy; another is to break style and form coocurrence or coowner. (These are statements of options, not endorsements.) There’s a middle course, though, which I do endorse: Revise references to co-occurrence (“simultaneous occurrence”) or co-owner (“fellow owner” or “part owner”) and preserve the ideal that co- constructions are always closed. (Consider, too, avoiding words such as co-conspirator, in which the prefix is extraneous; what other kind of conspirator is there?)
There are always going to be such challenges (“The co-op manager flew the coop”), but you, dear writer or editor, always have access to online and print thesauri and synonym finders.
Some people bristle at the sight of coworker, distressed by the thought that the word seems to suggest someone who orks cows. (Sometimes what wonders exactly what else, if anything, certain colleagues do during their workday.) I respect the aesthetic sense of the protesters; they are reincarnations of those who shuddered at the sight of, yes, cooperation (formerly co-operation — referring, literally, to operating together — then coöperation). But today’s upstart is tomorrow’s ubiquity.