I think I solved the problem:
I have used the UniqueFields-property of the TOracleDataset to identify the ID-field. It seems to work this way.

There's one thing I don't understand. The n-tier demo delivered with DOA does not use this UniqueFields-property.
With this demo updating is no problem. I checked all the properties in the demo and compared them to the properties in my project.
So there must be another possible solution to the problem. Any idea?