OpenCV was not made for OpenFrameworks. It is a project initiated by Intel Research many years ago (first version in 2000). The library written in C is optimized for real-time processing and takes advantage of embedded hardware acceleration. It runs on many platforms and is free for commercial and research use under a BSD license.
Oreilly’s has a good book for it if you are interested:
Learning OpenCV: Computer Vision with the OpenCV Library
http://www.amazon.com/Learning-OpenCV-Computer-Vision-Library/dp/0596516134
Many wrappers and implementations are now available, so OpenCV can be accessed from Python, Java, C# and many other programming languages.
Thanks for the correction. It looks like the Processing library of OpenCV is recent.