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

Update: Duncan Appelbe from the University of Liverpool in the UK also presented a talk at a JA-SIG UK meeting on July 10 2006 on their uPortal / Oracle eDiary integration using the Oracle SDK approach (see below).

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. Oracle themselves provide a couple of tools (both include Java APIs).

• The web services toolkit for Oracle Calendar uses Apache SOAP (not Apache Axis) and returns data in xCal format.

• A Calendar SDK which returns calendar data in iCalendar format (non-XML) and contacts data in vCard format (non-XML).

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).