Consider this session:
...
$ rm -f a.out b.out
$ gcc ~/hello.c -g
$ ls a.out
a.out
$ dwz -o b.out
$ ls b.out
b.out
$
...

The advertised usage looks like this:
...
$ ./dwz -?
Usage:
  ./dwz [common options] [-h] [-m COMMONFILE] [-M NAME | -r] [FILES]
  ./dwz [common options] -o OUTFILE FILE
  ./dwz [ -v | -? ]
...

But the actual dwz use we did above suggests instead:
...
  ./dwz [common options] -o OUTFILE [ FILE ]
...

So, we should either update the advertised usage, or require an explicit file when using -o.