The heartbreaking news of his death was shared by his aunt, who told Associated Press the young baby died of SIDS (Sudden Infant Death Syndrome).
'Oliver was happy, joyful and full of love, and he shared that with everyone he came into contact with', his aunt said. 'He was extremely healthy and was tragically taken too early by Sids.'
Oliver Jack Carter Lomas became an Internet sensation after a picture of him and Bernie Sanders went viral, in which he is dressed up like the presidential candidate at a rally in Las Vegas, in a white-haired wig, oversized glasses and plaid tie.
Babies for Bernie 2016— Susan Lomas (@SusieLomas) February 15, 2016
Oliver Jack Carter Lomas pic.twitter.com/8LISu2N8UW
His mum, Susan Lomas, spoke to LA Daily News about her loss and said Oliver came as a surprise to her at 45 years old, when she wasn't looking to get pregnant.
'It's going to hurt for the rest of my life' she said. 'I just have to count my blessings that I had him for four joyful months.'
She also shared details of Oliver's warm personality, that shone through the famous pictures.
'Oliver loved to be held', Susan said. 'I think the love I gave him, he shared with everybody who came into contact with him.'
The grieving mum has alo been honouring her baby boy on social media.
'I love you dearly, I love you most. I love you with all my heart and soul. I know that God will keep you near since God and truth are here', she wrote on a Facebook post.
Bernie Sanders is said to be issuing a statement to honour his young admirer, but a spokesman for his campaign said the situation is 'very sad'.
Oliver's funeral services are scheduled for next Tuesday in Los Angeles, where he was from.
Many of Oliver's fans have been paying tribute to the cheerful baby on social media.
Lindsay Radic wrote on Twitter: 'Prayers to the family of #berniebaby it's just another reminder no day is ever promised, even to the healthy.'
SIDS, also known as 'cot death', is the sudden, unexpected and unexplained death of an apparently healthy baby. Just under 300 babies die of SIDS in the UK every year, and most deaths happen during the first six months of a baby's life.
Our thoughts are with Susan and the rest of Oliver's family.