If you specifically wanted to find couples, polymatchmaker would probably be better than okcupid. (Except for the fact it's a smaller site with fewer members.)
With okcupid, if you see "Available" instead of "Single" as the relationship status, that's usually the closest thing to a poly flag you can get.
ALSO: Dig through the match questions. Be sure to answer any on polyamory and/or open relationships that they have. (As well as any regarding jealousy, etc.) Then rate them as Mandatory, or at least very important. Then when you look at another person's profile questions, click the option "Show questions I care about", to see how they answered them.
And of course you can say polyamory and polyamorous in your profile and do keyword searches for those who have done the same.