On his police bicycle, Sgt O'Brien carried out a thorough search of all the industrial units, car parks and wooded areas asking anyone he came across whether they had seen a person in distress.
It was during these enquires that Sgt O'Brien approached a group of ASDA employees outside their store in Marlowe Way.
It was then that the store's delivery staff raised their concerns about being unable to make a delivery to an elderly customer's address on two occasions.
Suspecting that the ASDA customer may need help, Sgt O'Brien made his way to the man's address in Ensign Way.
After obtaining a key from a neighbour, entry was gained and the man - 74-year-old Keith Lister - was found slumped in a chair in the living room of his ground floor flat.
"He was in a bad way and looked like he hadn't moved for at least a day or so," said Sgt O'Brien. "He was seeing with his eyes but he couldn't communicate verbally and was extremely dehydrated."
London Ambulance Service was called immediately and Keith was taken to St Helier Hospital for treatment.
Sgt O'Brien said: "Although we couldn't find anyone in distress in the area following the call to police, we did manage to help this man." Sgt O'Brien added: "We also helped to rescue his cat called Lucy, which was inside his address and also extremely poorly when we arrived. Clearly, the cat had not had had any food or water for some time."
Chief Inspector Safer Neighbourhoods Duncan McMillan said: "This incident underlines how important it is for members of the public to tell police what's going on - information which can make a real difference and change people's lives as it has done in this case."
Hospital staff reported that the man had a severe infection but was now said to be recovering well.
A senior nurse in charge of his care said: "If police had not come along when they did, this man who had a variety of issues, could have deteriorated further and died."
From his hospital bed at St Helier Hospital where he is recovering from pneumonia, Keith commented: "I don't remember police coming into my house. I was sitting in my chair. The next thing I knew, I was in A&E.
"I was really lucky and so grateful to the police for what they have done," said Keith, who used to have a model shop in Bromley and used to make props like tanks for films.
He said he had worked on the Dirty Dozen and met stars like John Wayne, Clint Eastwood and Richard Attenborough. He had also worked on some of the Carry On films like Carry On Doctor. His home is packed with film memorabilia.
Caption Keith with PC Steven Routledge, of Beddington North Safer Neighbourhoods Team