I really get the impression, that Oracle is a real mess according to work together with other software
Meanwhile I installed node-red-contrib-oracledb-mod and now the oracle node is visible. Unfortunately I now fail to force node-red (or the node) to use the tnsnames.ora. But this is another topic.
So: for my Windows 7 system from all available oracle nodes the only installable oracle node that also shows up in the UI is node-red-contrib-oracledb-mod