Mr O’Brien, 55, was attacked in the early hours of 3 December 2016 in a communal stairwell of flats at 12 Vanguard Way, Wallington.
Later that day Carl was taken to a south London hospital as a result of having lost consciousness. He had told a family member that he had blurred vision and a persistent headache after the assault.
Sadly, Carl, a cab driver, died from his injuries on 23 December. A post-mortem examination gave cause of death as a blunt trauma injury to the head.
The Met’s Homicide and Major Crime Command took over the investigation from Sutton CID following his death.
A 26-year-old man was arrested and initially charged with GBH with intent, and the charge was changed to murder following Mr O’Brien’s death. He was found not guilty of murder following a trial at Isleworth Crown Court in October.
Following the acquittal, the case is still open and detectives are appealing for any new information that may lead them to the person who carried out the attack and inflicted the fatal injuries.
The Met is offering a £20,000 serious crime reward for information that leads to the identification, arrest and prosecution of the person or persons responsible for Carl’s death.
Carl’s daughter Laura said in a family statement: “Our dad was an amazing dad, granddad, son, brother and friend to so many.
“He was a kind, generous man who wouldn’t hesitate to help anyone. Our dad was in our lives every day and we grew up knowing how much he cared and loved us.
“Dad was distinctive by his stature, generosity, sense of humour and heart. Lovingly named ‘Wurzel’ by many, nobody would forget meeting him and everyone has a funny story of him.
“Dad was a loyal friend and at his funeral over 600 of his friends attended and celebrated his life. As a staunch Chelsea supporter and ex-groundsman, football was his passion and he travelled far and wide to support them.
“In his honour on the 55th minute of the Chelsea v Hull match a minute’s applause was held. Everyone stood and applauded. Many knew him in person or by sight.
“A year later and the loss is real, his friends and family are lost without him, we miss him, we love him but we don’t have him anymore.
“This attack took place on his only granddaughter’s first birthday, a day he had been looking forward to. He never got to see her again. No words can be articulated or expressed to explain how we feel.
“Seeing the injuries that our defenceless dad sustained will haunt us for the rest of our lives. We sat with family and friends by his bedside for three weeks praying for him to come back to us, but the injuries were too severe and he passed away on 23 December 2016.
“At a time when most families were looking forward to their Christmas, all we had were tears, grief and sorrow.
“As the anniversary approaches, we would like to appeal to people to come forward with information relating to this attack so that the perpetrator or perpetrators are brought to justice. We are a normal family who never imagined that something so unjust, so unnecessary and so senseless would happen to such a gentle and well-loved person.
“We as a family wouldn’t want anyone to have to go through what we live with every day.
“We understand that not everyone wants to engage with the Police so please contact Crimestoppers with any information that you may have, it doesn’t matter how small. It could make a difference.
“Please search your conscience, look at the ones you love, imagine them gone tomorrow by the cowardly actions of individuals. Please help to get justice for Carl, our dad. Thank you.”
DI Jo Sidaway, leading the investigation, said: “Mr O’Brien died just two days before Christmas last year, leaving his family totally devastated. They are still grieving, and now face spending another Christmas without Carl and with his killer not yet identified or brought to justice.
“We believe that on the night he was attacked, Carl went to open the communal door to the block of flats he lived in after hearing his bell being repeatedly buzzed. After a brief conversation with the people at the door, he turned his back and was knocked to the floor and subjected to a violent, unprovoked assault, with those involved stamping on him as he lay injured.
“I am sure that there are people who know who did this to Carl, and that someone – most likely in the Sutton or wider South London area – has the information to help us solve this case.
“You may be fearful of providing information, or have loyalties that are stopping you from doing so. My message would be to do the right thing and come forward. Tou are protecting a killer.
“We are counting on you to get in touch, and we await your call to our incident room. We would treat any information provided with the strictest confidence.”
Anyone with information about Carl’s death can contact the Incident Room on 020 8721 4005, or via the police non-emergency number 101.
To give information anonymously contact Crimestoppers on 0800 555 111 or visit crimestoppers-uk.org.