Oppenau is situated in the Rench valley in the Black Forest. The nearest major cities are Offenburg and Freudenstadt.
Wikipedia

At the Kreuz Inn, near Oppenau, the road to Freudenstadt and Antogast (see below) diverges to the r. At (6 M.) the busy little town of Oppenau (*Stahlbad; Post) “Kirschwasser' is largely manufactured.
Bædeker - Rhine, 1873 page 255

See Bædeker Rhine 1886 page 361