Elland Road chiefs have struck a deal with the National League strugglers for the 18-year-old striker.
Dalby, who made his professional debut last season, has been booked in for a medical tomorrow.
According to Football Insider, he has agreed a two-and-a-half-year deal with Leeds and will sign the contract in the next 24 hours.
Dalby, who will initially link up with the club’s Under-23 side, has been earmarked as a future first-team star.
The Leytonstone-born hotshot scored on his full league debut for Orient in December 2016.
He went on to make 16 appearances in League Two for the club but could not prevent them sliding out of the EFL.
Dalby, who stands at 6ft 3ins, has been limited to two substitute appearances this term for Orient.
Leeds have been strengthening their youth ranks in recent weeks.
Polish prospects Bartosz Boniecki and Hubert Sadowski are due for a trial spell this week.
Goalkeeper Kamil Miazek, 23, put pen to paper on a short-term deal while forward Oliver Sarkic, 20, turned his loan move from Benfica into a permanent deal earlier this month.
Promising striker Ryan Edmondson, 16, was snapped up from York City on a long-term contract in November.