Installing OpenCV has never been easy and always required a lot of careful usage of the command line to build from source. This was especially painful when working with a Raspberry Pi as building and installing OpenCV took a lot of time on the RPi, especially on the older models. Luckily this has changed very recently as it is now possible to install OpenCV with pip!
Below I guide you through the basic steps that I think are necessary to get opencv to work nicely on Mac, Ubuntu and Raspberry Pi. If you want more background information, see the excellent article by Adrian Rosebrock from PyImageSearch.com.