The 'Mass of the Angels' took place at St Bernard's Church, Glengormley on Sunday ahead of a private cremation.
Over 1,000 people attended the service which was celebrated by Fr Damian McCaughan, who described Oscar as "a symbol of bravery and goodness."
During the Mass, Oscar's parents Stephen and Leona both spoke about their little boy's brave battle with neuroblastoma and the love he had for his little sister Isobella.
His dad told mourners: "Oscar meant the world and more to me and Leona.
"But in all honesty, there are no words that could ever do him justice.
"He was kind, caring, sensitive, loving, smart, mischievous and unbelievably funny.
"He had the most wonderful little personality and uncanny ability to wrap people around his finger within seconds of meeting him - particularly female nurses."
He ended his tribute, saying: "I love you wee man. Best muckers forever."
He has achieved much more in his five and a half years than many of us could ever hope to achieve in a lifetime.
Oscar's white coffin was decorated with the characters Peter Pan and Tinkerbell from the child's favourite film.
His mum Leona told the packed church how she explained to her eldest child that he would soon be going to heaven.
"We talked to him a few weeks ago about going on a journey to Neverland. We explained in Neverland there are no sore knees, no sore heads and no sore tummies.
"Oscar had always told us that when he grew up he wanted to drive a big green tractor instead of a car and he was so excited to hear that Old McDonald lives in Neverland and allowed people to drive his big green tractor if they were five and a half.
"The best thing about Neverland was the biggest dodie [dummy] shop in the world and that you got a new dodie every day," she continued to laughs in the crowd.
When we got to this point in our story, Oscar took his dodie out of his mouth, had the biggest smile and said: 'This is the best day of my life'.
Both tributes were met with standing ovations from the mourners.
Among those paying their respects were Deputy First Minister Martin McGuinness, Belfast boxers Carl Frampton and Paddy Barnes and award-winning restaurateur Michael Deane.
After Oscar was diagnosed with the disease at the age of two, the Knox family mounted a social media campaign in a bid to raise funding and awareness for treatment.
In April last year, Wee Oscar was given the all-clear, but sadly the cancer returned a few months later and he passed away on Thursday afternoon.
Wee Oscar was a Celtic fan and the club players wore black armbands as a mark of respect during Sunday's home match with Dundee United. A tribute was also shown on big screens before the game at Celtic Park where fans stood to sing You'll Never Walk Alone.
On Sunday night, Belfast also paid a special tribute to the brave boy, as the city hall was lit up in blue and yellow - the colours of #TeamOscar - in his memory.
His dad tweeted: "Today was difficult, yet easy all at the same time. Thank you all for your support. A great send off for our wee superhero."
Over £30,000 has been raised through a special Just Giving page after the Knox family asked for donations to be made to Northern Ireland Children's Hospice and the Haematology Unit at The Royal Belfast Hospital for Sick Children in lieu of flowers.