I would like say what are the best sources to learn OBIEE and where to find them…
First of all, you should know what is Business Intelligence and you should be familiar with BI terminology.
Once you are OK with this, you can start learning from Oracle by Example tutorials. These oracle tutorials are great source to get basic to advanced knowledge.
Once you finish Oracle tutorials, you can start reading other blogs like the once that are in Blog roll section or just google it, you will find many of them. But I highly recommend you to go through Oracle tutorials first.