O'Ward, who drives for the Arrow McLaren IndyCar team, is in Abu Dhabi and will participate in Friday's first practice ahead of Sunday's season-ending grand prix.
The Mexican has four wins and 20 podiums in IndyCar and has previously tested with McLaren, including last year in Abu Dhabi.
"Pumped to take on this new role within the McLaren Racing family," said O'Ward in a statement. "I’ve always said it's never a bad day when you get to jump into an F1 car, so I look forward to joining the reserve driver pool for next year alongside my driving duties with Arrow McLaren.”
O'Ward has been on the F1 radar as a promising talent for some time having also had a stint with the Red Bull junior team.
He joins a reserve squad that includes Japan's 24 Hours of Le Mans winner Ryo Hirakawa (29) and Mick Schumacher (24), the son of seven-time F1 champion Michael Schumacher.
"Pato had an impressive season in the IndyCar series and has performed well in his driver development tests, so it felt natural to take this next step now he’s eligible for an FIA Super License," said McLaren Formula One team principal Andrea Stella (52).
"With the expansive calendar, it’s prudent to ensure we have a wide pool of drivers that we can call upon if required."
There are unlikely to be any open seats at McLaren in the near future with the team having two of the series' top young drivers in Lando Norris (23) and Oscar Piastri (22).
