The Oklahoma Genealogical Society has published "Index to Probate, Oklahoma County, Oklahoma, 1895-1920".The introduction says this: "The District Court Clerk, Oklahoma County, Oklahoma keeps a series of books in the Probate Section titled 'Probate Index 1' through 'Probate Index 6'."[Book 1 begins with 1895 and goes through 1949.]"All cases filed prior to 1895 were indexed as 1895.There were some cases files as early as 1889."
The entries for Douglas are not earlier than 1908 - your 1899 death should have been in Book 1.Write to the courthouse and ask them to double check to see if there is a probate record in the indexes that was overlooked in the publication.
The address is for the Oklahoma County Courthouse is 320 Robert S Kerr, Oklahoma City, Okla73102
Canadian County probate records are shown, in the Handy Book, as starting in 1900 - later than you need.