crossy wrote: The spreadsheet is not the source of the labels any more, the ODB file is.
It is important to know which file contains your data actually.
Open the database document (*.odb).
If you see this on the status bar...

... then your addresses are saved within that document. The document embeds a HSQL database.
More likely it looks like this ...

... which indicates that the source of your data is a spreadsheet. The Base document makes this spreadsheet look as if it were a database. This is called an "abstraction layer". Writer always reads from a database, however behind that database you may have a spreadsheet, a text file, a dBase file, some mail address book, a true database of a third party vendor or a database that is embedded in the Base document.
[any kind of database-like data] ---> [Base document] ---> [Writer document with mail merge fields]
This way you define your source of data once and re-use it many times for all kinds of letters, labels, envelopes or other documents with variable content to be read from a row of some table.
With a well prepared Writer template, writing the same professional letter to hundreds of receipients is a matter of minutes.