There were a number of Oldenburgs in the 1928 Fond du Lac city directory, browseable thru Distant Cousin's website. Here is one page:
distantcousin.com/images/Directories/WI/FondDuLac/1928/313.jpg
Several possibilities for Raatz or Russ: Rasch, Rusch, other R's, in this same directory:
http://distantcousin.com/Directories/WI/FondDuLac/1928/

The name Oldenburg might be in FDL church records. Were any of them possibly members of the Church of Peace? It had a German alternate name, Friedens something?