"Al-Kass will succeed Mohamed Helmi, whose contract has expired, as Olympic coach," the club's board Ibrahim Al-Sheikh told FilGoal.com.
A poor season saw Olympic suffer relegation to the Egyptian second division after just one season in the top flight.
Former Egypt striker Al-Kass quit his job as Gaish assistant coach in order to take over at Olympic, the club where he began his career.
"This club is my home. They will find me available whenever they need me," he told FilGoal.com.
Al-Kass' task will be helping Olympic return to the Premier League next season.
A series of financial problems prevented the one-time Egyptian champions from being active in transfer windows.
They failed to bring in quality players last summer to finish bottom of the 16-team league table.