AskTom is a great site that I dip into pretty regularly. Say what you like, but you’ll never find Tom Kyte without an opinion or an illustrative example on how to do things the right way. Except when he doesn’t have an opinion or experience, but he’s refreshingly open about that also. His examples are clear and concise (and your own had better be also), and it’s a rare day that you can’t learn something new.
You’ll sometimes find me hanging around the forums at DBAsupport.com, answering more than asking, I guess. The Obfuscation Unlimited forum is usually lively, though less so now that the election is over ;)
The Oracle Technology Network has it’s own forums, which I have mixed feelings about. They’re busy, but the interface is not my favourite, nor is the general style and content of the questions. However, if you’re an Oracle professional and you don’t have an account at OTN then you’re missing out on the best source of information available — the documentation site. Get that on your bookmarks, because 90% of the questions that you see on almost any forum can be answered with a one minute search of the relevant documents. The Concepts Guide, which is essential reading, runs to 732 pages in PDF format, so that gives you a feel for the level of detail available. At the very least it should be browsed so that you know that the information exists, even if you can’t absorb it all.