Somehow for a lot of people OpenCV goes very bad on macOS. I've created a github gist that will help you set it up properly on macOS. The issue i think arises when setting up the environment variables properly.
Here is a link to the gist. Please feel free to provide your feedback or if i might have missed anything. Will surely incorporate that in.
Hope this helps you set it up properly! Make sure to give it a start if it helped.