Thorson/Ois/Thoresen?

I am searching for information on my gg-parents, Otto and Mary Thorson, who came from Norway (1880 and 1881/4 respectively). In the 1900 Census for Michigan, they used the surname "Ois" and after that used the surname "Thorson". Some of the children used "Thorsen", but most used the spelling "Thorson". They had at least two children born in Norway, Charles/Charley about 1879 in January and Josephine about 1881 in April, children born in the USA were Fred, George and Anna and there was also a child named Amelia of which we have no information, except that she was a child of Otto and Mary, information from a living child of Fred Thorson. The Census records show that Mary had 6 children, but only 5 were living, we have been unable to find if she actually had 7 or 6, as we know of 6 children that were living. In the Digitalarkivet I have found an Otto Thoresen traveling to Big Rapids, MI in 1880, Big Rapids is the area my Otto and Mary settled in. I also found an Otto (birth place, Eidskog) and Marie (birth place, Ullensaker) Thoresen listed in 1875 in Kristiana and Otto Thoresen in 1865 in Eidskog. The ages are correct for all of these findings, but not knowing Norway, I am unsure I have right Otto and Mary!! On Otto's Death Certificate, his father is listed as "Torrey Olson" and his age at death was 64 years, 10 months, 0 days on March 22, 1913 (date of death). In Census listings, his birth month was listed as March and year of birth as 1849. We have been unable to locate a death certificate for Mary at this time, but she records her birth month as December and year as 1845 in the Census. On some of the children's birth and death certificates, Mary is listed as the mother using the surnames of "Ois", "Olson" and "Oleson", except for the death certificate of Charles, the mother is listed as "Unknown". Any help locating parents, places and years of birth, years of immigration to the US, marriage of Otto and Mary, or any other information would be greatly appreciated!!!!!!!! Thank-you.