The striker will arrive on a three-year deal after finalising personal terms and passing a medical at the Amex.
The 28-year-old could make his first appearance for his new club in Wednesday's friendly against Crawley Town and boss Sami Hyypia is delighted to have bolstered his attacking options.
"This is an area we want to strengthen and Chris is a good start," Hyypia told the club's official website.
"He is a strong, physical presence and gives us something different to the other strikers we already have here at the club.
"You need that option in the squad of a forward with power and strength, and Chris can give us that - as well as scoring goals.
"We still want some extra attacking additions, and in other areas of the team, but I'm pleased we have another one of our targets."
The deal for O'Grady will be a major boost for Hyypia as Brighton are understood to have accepted a £7millon bid from Leicester for striker Leonardo Ulloa.