Finally I've solved this.
There were two things wrong.
(1) I've configured the qt build with -qt-sql-oci and -plugin-sql-oci. There can only be one of them, the second one overrides the first. That's why QOCIDriver has not been included in QSql4
(2) The wrong oci.dll has been loaded. There was a copy of oci.dll (win32) in my debug folder. Don't known how it got there, but in any case this one was found before checking the path I've set in the debugging environment. I found that in the output of VS2010, where a list of all drivers loaded is shown.
Some general tips for comiling qt with oci:
check if the oci.lib and oci.dll have been build in the same mode (x64 or win32). You can do that in a VisualStudio command prompt with DUMPBIN /headers xxx.xxx
configure the include path by using -I D:\Oci64\includes (or where ever you store these files
configure the library path by using -L D:\Oci64\lib
That's it.
Greetings,
Harald