Oak is broken up into functional packages that can be installed with bower.
npm install -g bower
bower install oak
This will install the oak core package. If you want to use any of the other packages you must install them separately.
bower install oak-animate
bower install oak-canvas
bower install oak-data
bower install oak-dom
bower install oak-router
bower install oak-tween
...
Alternatively the minified source for each package can be found in its' repositories build directory.