Italian Fornaroli, 21, joined McLaren last month in a test and development role and will continue those duties supporting Britain's world champion Lando Norris and Australian teammate Oscar Piastri.
Mexican O'Ward, who will continue to race in US-based IndyCars, was also a reserve last season.
Dutch driver Richard Verschoor, third in F2 last year, will compete in the European Le Mans series and test for McLaren as the team prepare to enter the hypercar category of the World Endurance Championship in 2027.
The Formula 1 season starts in Australia on March 8th.