Before you start using OBS, you'll probably want to learn the basics of RPM packaging. Duncan Mac-Vicar has written a good tutorial that we suggest beginner packagers review before diving into OBS. Fedora offers another good resource.
The OBS is documented in this wiki. Packagers should start reading the tutorial and the cross distribution package how to. Some general hints can be found on the FAQ and Tips & Tricks pages. There are various presentations available. An important piece is the documentation for the API. The API is a simple and generic interface to work with the Open Build Service. You may use it to integrate your tools to submit code updates, check the current state of your projects or doing something very different and cool.