Well, NeXTSTep had DBKit which had an Oracle adaptor, there's no reason Apple couldn't have great native Oracle support very quickly if they needed it.
There's JDBC thin and Instant Client too. There's even a version of the Oracle server itself, but it's a few revs behind and not supported for Production use. If you wanted to write an Oracle app on OSX today, there's nothing stopping you.
The Oracle client libraries (in both traditional and "Instant" variants) for OS X on Intel weren't released until spring of '08. That was nearly a two-year wait relative to when many, if not most, developers using Apple hardware had moved over to the new hardware.
So yes, technically, there's nothing stopping you from writing an Oracle-based app on OS X today, but the story was quite different just a few months ago.
Also, while CoreData may have taken a lot of the nicer bits of WebObjects' ORM tier and re-packaged it for desktop apps using SQLite, the story for traditional client/server database applications isn't quite so compelling. RubyCocoa + ActiveRecord (or your preferred scripting language w/ORM and Cocoa bridge) actually seems like a much better option than bare Cocoa + Oracle C client libs.
There's JDBC thin and Instant Client too. There's even a version of the Oracle server itself, but it's a few revs behind and not supported for Production use. If you wanted to write an Oracle app on OSX today, there's nothing stopping you.