The "unanticipated" Triple 8 zone was discovered about 1500m into the Deep Underdog hole, which is still in progress and aiming for its primary targets in the outer shell of the porphyry intrusion about 2500m from surface.
The above-mentioned drill hit was in a wider 28.3m intercept from 1491.3m grading 20.4g/t uncut.
"We believe we have a tiger by the tail," president and CEO John Burzynski said.
He said the Triple 8 zone was more than 600m from any previously known gold mineralisation and underscored the view there was room for the Windfall deposit to grow.
Osisko has two deep exploration holes underway, Deep Underdog and Deep Lynx, testing for depth extensions and to further test the intrusion-related geological model.
The company announced its maiden resource for Windfall in May, labelling the indicated 601,000 ounces and inferred 2.28Moz "a great start".
Osisko spent about C$32 million (US$24 million) on exploration and evaluation assets in the March quarter and ended the period with more than $150 million (US$113 million) in cash and investments.
Its shares touched C$2.10 intraday but eased to close up 6c at $2.03 yesterday, towards the lower end of a 52-week range of $1.61-$5.07.