O'Leary, a horse owner himself, had planned to offer the money as a prize to the owner of the winning horse in the race before the Irish Grand National at Fairyhouse on Monday.
However, when his own horse won that race, he instead decided to give the sum to McNamara who will never walk again after fracturing two vertebrae in his neck last month.
Lisa Hancock, the chief executive of the British-based Injured Jockeys Fund, said the donation was "fantastic" news.
"I know that it will mean a huge amount to JT's wife, Caroline, and the family, just to know that the whole industry is rooting for them.
"It's a dreadful injury and clearly it's going to be a long haul from here but this is a magnificent gesture, the kind of thing that makes you proud to be involved in racing."
McNamara escaped brain damage after his fall and is fully alert but requires full-time ventilation. He is expected to be transferred to the National Spinal Unit in Mater Hospital, Dublin, in the coming weeks. His wife and three children are all based in Ireland.
The 37-year-old was riding Galaxy Rock for trainer Jonjo O'Neill in the Fulke Walwyn Kim Muir Challenge Cup on Wednesday of the Festival when the pair fell at the first fence.
An air ambulance landed at the racecourse to treat the injured rider, who was conscious after the fall but had difficulty breathing, and he was taken to the hospital.
As an amateur rider McNamara's career included four winners at the Cheltenham Festival.