For over 5 years now I have been reading Tom Kyte's AskTom site hosted on Oracle's site. This is the first place I go when I have an Oracle related problem.
For the past 2 years or so, I have also been reading his blog.
The second source of information concerning Oracle is Metalink. I mostly find all my answers on AskTom, but for the more obscure problems, this is the best place to go.
Third would be the OTN forums, mainly the SQL and PL/SQL forum.
The C.D.O.S. group is next followed by a general search on Google. Most of those lead to Morgan's library on the PSOUG site.
Of late, I have found many blogs as well. Particularly Dratz, who has a very informative series on How Oracle Really Works. We've exchanged a few emails over the past couple of months and I believe we have a similar belief about the use of the database (particularly Oracle).
Lewis Cunningham is another good source of information. He talks about Oracle, EnterpriseDB and PostgresSQL. He is currently utilizing Streams (Advanced Queueing), has used XML in Oracle and is/was a database architect. I have been reading his blog for a couple of years now.
I also read Howard Rogers, David Aldridge, Mark Rittman, and Jonathan Lewis (though I don't understand much of what he posts).
APEX - Application Express (formerly HTMLDB)
The OTN forum is my first choice. I contributed quite a bit there before moving to Tampa late last year.
John Scott - He's a big contributor to the APEX community. In fact, he received the Oracle Ace award last year. Mainly though he runs a site called shellprompt which allows you to host your APEX application. I was able to utilize his service for about a year with my previous business and I still have a site hosted with shellprompt. John's service was second-to-none!
Patrick Wolf - I've been reading his blog for a few months now, but unfortunately, I don't work with APEX (at work) anymore.
Carl Backstrom - He works on the APEX team at Oracle. His sample/example stuff is excellent.
37Signals - Very transparent business that posts about how they run a business, their development philosophy (getting real), and design decisions. I have used their Ta-da List (to do list) and Writeboard (collaborative writing).
I'll continue to add more links here as time goes on; or you can just check out my delicious links to Oracle.