Smith, 45, and O'Kelly, 60, will now remain at Griffin Park until the end of the 2018-19 season.
The pair joined the Bees from Walsall in November 2015.
"It is great news for the club that both Dean and Richard are fully committed for the long term," co-director of football Phil Giles said.
"Their arrival helped us bring some stability and allowed us to concentrate on building some solid foundations for the future."
Smith has won 23 of his 62 games in charge of Brentford in all competitions.
Brentford head coach Dean Smith and his assistant Richard O'Kelly have signed one-year extensions to their contracts with the Championship club.