The 53-year-old Portglenone man claims it was "never my intention" to let anyone down and says he will carry the shame of what happened "for the rest of my life".
In the letter to Belfast Crown Court judge Mr Justice McLaughlin, O'Kane also said: "It is with the deepest regret that I find myself here before you today."
At the end of his trial last month, the businessman was convicted of 25 charges of obtaining money and property by deception - but he later pleaded guilty to a further 126 counts and two charges of fraud by false representation.
On Friday, prosecuting QC Liam McCollum revealed that there were a total of 59 victims and that the total money they lost was £3,983,735.
O'Kane has already been warned that a jail term is "inevitable".
Mr Justice McLaughlin has also asked the victims to agree to their impact statements being given to the fraudster, to give him an insight into the extent of what his actions have done to people.
The judge revealed that one victim, who handed £75,000 to O'Kane, has been told by their bank that "literally they will be taking £600 a month for the next 17 years".
The charges O'Kane has been convicted of arose after he portrayed himself as the landowner, builder and developer of the Golden Beach development in the Turkish sunshine resort of Bodrum.
But, as one victim testified, the conman "didn't even own a blade of grass".
Defence QC Brian Kennedy claimed his client had been physically attacked by masked men on two occasions and further claimed he had "foolishly trusted" his business partner, been made the fall guy and left "in the dark" as to where the money had gone.
Mr Justice McLaughlin accepted that he "wasn't dealing with one of life's great rogues" - but told Mr Kennedy that, while there were people in the dark, "Mr O'Kane wasn't one of them".
O'Kane was remanded into custody until his final sentencing next Friday - but with the judge already stating that he will be jailed for his crimes, the only question is how long he'll have to serve.