Dear Lily White,
French census have been written for each city / town / village, every 5 years, except during World Wars I and II, off course. So, regarding the period of time you are interested in, census of year 1911 could be useful, and maybe the 1921 census as well. Depending the geographical area you are looking for, census may be available online, from the Archives Départementales web site. Do you have an idea where William Charles Otter lived in France ?
Hopefully I could help you a little bit.
Greetings from France !