O'Ward already has F1 experience after taking part in several MCL35M driver development tests, as well as driving in the 2021 Abu Dhabi young drivers test, and FP1 at last year's Abu Dhabi Grand Prix.
The 24-year-old, who will take part in first practice for this year's Abu Dhabi event, as well as next Tuesday's post-season test at the Yas Marina Circuit, said: “Pumped to take on this new role within the McLaren Racing family.
“I've spent a lot of time with the F1 team having driven previously in free practice and taken part in a few tests. 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. Thank you to Andrea and Zak for this great opportunity.”
Team principal Andrea Stella said it “felt natural” for O'Ward to take “this next step now he's eligible for an FIA super license.”
Stella added: “With the expansive calendar, it's prudent to ensure we have a wide pool of drivers that we can call upon if required. We look forward to seeing him progress in this new role.”
O'Ward joins Mick Schumacher, Felipe Drugovich, Stoffel Vandoorne, and Ryo Hirakawa as part of the reserve pool.