Yes, OKCupid is very much a great place for polyamorists to meet other people open or already practicing poly. But that doesn't necessarily mean that you can create a profile there as a couple, since poly isn't automatically about couples finding thirds, of course. It is generally recommended and accepted that those already in committed relationships each have their own profile, since a couple is comprised of two individuals each with their own personalities, likes, dislikes, etc. It does help with the questions designed to find matches, as another poster already stated.
Many polyfolk will link to their partners' profiles at the top of their own and/or include pics of them together. If you are looking as a couple for someone to only be with both of you, your choices and responses will be much more limited than if you date separately but you have more of a shot at OKC than most other places.
Also try polymatchmaker.com - I believe they have a couple option for profiles (I'm not certain about that, though).
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein