thanks for your help. My answers/comments here below.
a) ORACLE_HOME not set
[MM] True. In the first run this variable wasn't set. But then I fixed this problem and I restarted the server.
b) incorrect path:
[MM] This a real problem. It seams that Timezone package requires some changes. Is this an Oracle or a Windows vrs. Linux problem?
(c) tsearch 2
[MM] Correct, my mistake. No need to install this.
[MM] This is another real problem. Categories seem to be working with Postgresql while they don't with Oracle.
The end result of points b) and d) is that XoWiki shows some error messages (though not directly generated by XoWiki itself).
The problem I see is that not all the packages/extensions work in the same way when running on Oracle or on Postgresql.
Cheers and once again many thanks,