<p>I need to do a simple mail merge in OpenOffice using C++, VBScript, VB.Net or C# via OLE or native API. Are there any good examples available?</p>

You should take a look at
Apache OpenOffice API
. A project for creating an API for Open Office. A few languages they said to support are: C++, Java, Python, CLI, StarBasic, JavaScript and OLE.
Java Example of a mailmerge in OpenOffice
.