Thanks guys for the help so far.
But the report builder doesn't seem to work. I have downloaded it, I installed it (from hard disk, not directly from then net), it then shows up in the extensions manager. At first I thought "maybe OO needs a restart", so I closed it, started it again, to no avail.
I open my DB, right click on the report, choose "edit", the edit dialog opens, and there is still only a "Navigator", no "Report Navigator". I then checked whether just its icon was deactivated on the standard toolbar, but nothing. I check all the menus, there is no trace of a Report Navigator anywhere. Except for the extensions manager. It's still listed in there.
Btw., just for having a look at the web way, I did launch "download additional extensions from the net", which loads
https://extensions.openoffice.org/getmore?cid=920794, and there the Oracle Report Builder only has a score of 75% on compatibility with OO 4,x...
I installed the "latest" version, 1.2.1. Btw, my OO is the 64 bit build in Debian file format (.deb).