Hi Ingrid, I found the Olsson Slakttrad Family Tree which contains this family, trees.ancestry.com/tree/18701308/person/698878193 .
It shows that Olov died 2 July 1951, Sacramento, California. I think his obituary will help you. There is an obituary volunteer at freepages.genealogy.rootsweb.ancestry.com/~obitl/ and scroll down to Sacramento Co.
OR Do a Google search for the Sacramento County Public Library.
His wife was marion C. Dardis b. 13 JAN 1918 WHO DIED 11 Nov 11, 1978, Beach, North Dakota, USA. You can post on the North Dakota Message Board and ask how to find her obituary.
Roland Patrick Olsson b. 29 June 1940 Mower, Minnesota, USA who died Oct. 24, 2002 in North Dakota. The tree doesn't say wher but in 1984 he was living in Dickinson, North Dakota.
Another son Robert James Olsson b. 28 March 1946 Mower, Minnesota, USA died 1988 but tree doesn't state where.
They had one other child who is still living. Successful Searching!