The Oxford Mail reported Thomas Yuill's remains were found at his Oxford flat when police forced entry on Friday.
Housing officers visited the property in January but saw "no concerns" for the 82-year-old's welfare and left.
Blackbird Leys councillor Linda Smith said: "I'm shocked...why wasn't [this case] followed up for so long?"
Mr Yuill stopped paying the rent on his Blackbird Leys flat in November, and housing officers visited his flat in Sorrel Road in January but saw "no concerns for his welfare".
In April, Oxford City Council housing officers called the city's John Radcliffe Hospital, county's coroner's office and care homes in the area to try to locate him.
On Friday the council asked police to force entry to the property, where Mr Yuill's body was found.
Cabinet member for housing Mike Rowley said: "It wasn't an unusual thing for his neighbours not to see him for weeks at a time."
He said the council was launching an inquiry into the "tragic case."
The council said it had identified 700 residents who might be "socially isolated" who would be contacted by officers in the next three weeks.
A man's body was undiscovered at his flat for seven months after he died, despite council officers visiting the property when he stopped paying rent.