These people have some knowledge of integrating Oracle Calendar with uPortal:
George P. Pipkin
Mark McLaren
Steve Barrett

Summary

You can pull data from an Oracle Calendar Server via web services. There is at least one project that interfaces with Oracle Calendar using a PHP client. Also, Oracle themselves provide a web services toolkit for Oracle Calendar which uses Apache SOAP (not Apache Axis).
See e.g.:

http://download-west.oracle.com/docs/cd/B13866_02/calendar.904/b10893/part2ws.htm

So in theory it should be possible to interface an uPortal channel with Oracle Calendar, although you would also have to deal with Oracle's Single Sign On mechanism somehow.

There is also a command line utility called UNIICAL which can be used to import iCAL objects into the calendar server (found inside Oracle Calendar Reference Manual).