The Oscar W. Pearson that died in Marion County, Oregon in 1947 was born in 1885... There is not enough information to clearly eliminate him
http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GSln=pe...;

When you post, it is important to provide a date of birth or best guess, or at least age as found in the 1940 Census. You may have the 1940 Census, but volunteers would have to look at it to determine ages of all four.

The 1940 Census of Mandan, Morton Co, ND
Oscar W. Pearson, age 41(as transcribed, 1898/1899) , born in ND, employment not really needed to research
Mable M (provider of information), age 47(1892/1893), born in ND
Wallace L, age 17(1922/1923), ND
Ronald W, age 8(1931/1932), ND
--The 1930 Census identifies Oscar was 41 at that time (1888/1889), born to both parents born in Sweden. Mabel's parents were both born in Denmark.

--The World War I Draft finds Oscar William Pearson at Mandan, Norton Co, his wife was Mabel M. Pearson. It shows his DOB as being March 11, 1885 (I find the year of birth on these draft registration to be off). BUT, if Oscar WAS born in 1885, the death in Oregon has a potential.

The World War II Draft (Old Man's Draft)
Oscar William Pearson, Manette, Kitsap County, WA. Born Mar 11, 1889 in Sims, North Dakota. Name and Address of Pearson always knowing his address: Mrs. Annie Miller (identified as mother of Mable), Lynden, WA. (The fact that Mable was not listed is NOT an indication of divorce, her death or other factors...this is found often). He was employed at the Puget Sound Navy Yard, Bremerton, Kitsap Co.
---THIS record shows his birth in 1889, which matches the US Census records.

Three family trees identify Oscar William Pearson was born 11 March 1889 and died in 1948 at Bremerton, Kitsap County, WA. His wife was Mabel M. Miller, born 1 May 1893, died March 1983 in Fruitland, Payette Co, Idaho. Ronald William Pearson died about 1993 in CA. Wallace Lynn Pearson, born 27 Nov 1922 in Mandan, died 15 July 1977 in Amarillo, Randall County, TX.
--These family trees also use the DOB of 1889; BUT, I find family trees are often wrong. One person posts information which isn't proven...just like the other poster here, and soon that wrong information is everywhere.

Wallace's grave:
http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GSln=pe...;

His wife's:
http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GRid=97...
THIS would probably be your best bet to find information on this family . It identifies she had a son, Mike Pearson of Amarillo, and others. I'd suggest contacting members of this family to see what information they had on "Grandma and Grandpa." www.zabasearch.com shows a Michael Pearson in Amarillo and a Patricia Kendrick in Wichita Falls.