As of April 4, the province will let private gaming websites that have qualified through a licensing process start taking wagers.
Rivalry, which has a gaming licence from the Isle of Man and already operates in other jurisdictions, said it has applied to become a licensed operator in Ontario.
Great Canadian, which is owned by U.S. private equity fund Apollo Global Management and operates the Woodbine and Pickering casinos among others, commissioned a report late last year that concluded the new regime would lead to a loss of tax revenue and casino jobs.
NorthStar Gaming also welcomed the news Friday.