Can I put 19 inch tires on my 2021 Outback

You sure can, just make sure the wheels will have the correct bolt pattern, offset and center hub diameter. You'll want to also purchase tires sizes 225/ 55 19

I wouldn't. Does your onlyx have 17 or 18 currently? The best handling upgrade on 18" wheels is to go to 225/55VR18. It'll sit only 1/4" lower, but clearly handle better. Comparable 19" wheel would need 225/50VR19, which will be TOO stiff for the native sloppy, rolly suspension. Unfortunately the 2015+ STi thicker antiswaybar will NOT fit on the 2020+ Leg/OB, and there's no improved aftermarket part available yet...if ever! But 19's won't help you. Stiffer springs and struts will, but that's a crap shoot and of course pricey. 225/55R18 and even 225/60R17 are fine improvements over stock, but it's the swaybar that REALLY improves handling. You'll also probably bend 19" wheels, so save yourself the heartache, eh?

Style over function. Gotta have those rubber band tires to look cool.