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If you didn't hear the latest news, Oracle 10gR2 for Mac OS X is officially available for download.
Thanks to Scott Spendolini for reporting this exciting news.
Awaiting to finish the download and to begin the installation on Mac OS X 10.5.6.
Updated April 14. If you are primarily interested in getting Oracle Application Express to work on Mac OS X natively, then you'll have to wait until Oracle releases the missing server components as Apex is not yet supported. See Scott's posting.