Hi, The Frank Coates family was living in Crescent, Del Norte Co., CA in 1910. Frank H. Coates, b. OR, 39, marr 11 yrs, teamster draying. Orpha, wife, b. OR, 31. Arlane, b. OR, 10. Milton, b. OR, 2. Merton, b. CA, 2/12ths.
I found an Orpha Myrtle Coates who died 1 Feb 1919 in Josephine Co., OR per death cert #12. Source: Oregon Death Index 1903-98.
There are a couple of obituary volunteers in Josephine Oregon in Random Acts Genealogical Kindness. Go to raogk.org Choose Oregon Scroll down to Josephine Co. and choose ONE.
Even if this isn't your Orpha it looks like she left or died before 1920 when Merton and Milton are in the orphanage in Beaverton.
Frank in 1920 was living with Charles and Arlene Wells in Portland, Multnomah, Oregon. Charles O. Wells, b. KS, 35, stacks briquettes gas plant. Arlene, wife, b. OR, 19. Frank H. Coates, fatherlaw, b. OR, 48, widow, briquett presser gas plant.
I didn't find any additional info for Frank in Ancestry or FamilySearch.
Have you requested a copy of Merton's obituary? It might mention Frank. You can request a copy from the Tulare County Library, tularecountylibrary.org/obituaryrequests.html .
A Merton Coates Born: 15 Feb 1910, OR Died: 4 Oct 1980, Tulare Co., CA Source: California Death Index 1940-97. Successful Searching!