I found William Riley in the 1900 Lane County OR Census, where he said that he was born in Indiana and that his parents were born in Kentucky. Interestingly, Rhoda appears in a separate entry on a different page with what I take to be their children. I note that she says that all of the children who were still at home were born in Washington and that both parents were born in Missouri. This would seem to indicate that William's parents were born in Kentucky c1800, moved to Indiana where he was born, and that they may have then moved to Missouri before he headed west. This is a long shot, but you might want to look more closely at an entry in the 1850 Census for Washington Township, Morgan County IN: William Hollinsworth 38 born KY Susannah37 IN Charity16 IN Presley14 IN Jeremiah12 IN Catharine10 IN Joel8 IN Sarah6 IN
I wonder if Presley is some garbled version of Riley, and that William went by his middle name to distinguish himself from his father.