The following command works as expected ...

% ofd2000 --call_graph -o=stack_calls.xml file.out

When the -o option is at the end of the command ...

% ofd2000 --call_graph file.out -o=stack_calls.xml

... the output is not written to the file named with -o, but to stdout instead. It ends with ...

error: failed to read "-o=stack_calls.xml"