Osborne was a sheriff in Ankeny, Iowa. He and his officers arrested Sam and Dean Winchester for the crimes committed by Leviathan Sam and Leviathan Dean. He came to believe in Leviathans and helped the brothers before being killed by Agent Valente.
Leviathan Sam and Leviathan Dean were on a killing spree to expose the Winchester brothers. Now on the FBI's most wanted list, the brothers were apprehended by Sheriff Osborne in Ankeny, Iowa. He was not inclined to believe the two suspected murderers' stories, but does grant Dean his one phone call. Dean used the sheriff's phone on speaker to call Bobby for an update. Bobby gave him a gruesome method to stop the Leviathans that disgusted the sheriff, who cut the call off.
When the sheriff was leaving the cell area, he saw a Leviathan eating one of his men and believed the brothers. He retreated quickly, as he realized the truth and released Dean, who instructed him to find products containing borax. Dean delayed Leviathan Sam long enough for the sheriff to douse the creature in cleaning fluid before Dean beheaded it. After Leviathan Dean was also decapitated, the sheriff let Sam and Dean go, and covered up for their disappearance by claiming they were dead to the FBI, and that he had the bodies cremated. His daughter, the medical examiner, supported the lie.
The FBI leave, but Agent Valente returned. He was a Leviathan, and killed both the sheriff and his daughter.