The address 7632 Mallard Dr is in St Louis County, not the city of St Louis. In the 1940 census Obed Janzen is indexed as Obed J Jangen, living in state of Arkansas, White County, Incorporated Place: Searcy, Township: Gray. Living with him is his wife Josephine. He lists his 1935 location as Joplin, Jasper County, Missouri. Josephine lists her 1935 location as Ottumwa, Wapello County, Iowa.
Find-A-grave shows a Lina Janzen, died Oct 22, 1936, buried in Joplin, Jasper County, Missouri, age 55. This could be Obed's mother.