I've re-used the o ring when it's in good condition. I don't re-use head gaskets but for some strange reason I hang onto them if they come off the head in good condition. It's kind of like keeping that worst pair of underwear because....well you never know when you might need them in an emergency.
I have several restored bikes along with a 2006 Goldwing with a sidecar. My wife has a 2019 Suzuki DR 650 for on and off road.
The only gasket I have ever reused is the Z1, Kz900, and early model Kz1000 valve cover gasket. Of all the Kz900's/Kz1000's that came down with a head gasket leak I have looked at it always seem to be the that center O-ring that started leaking. I never reused one on any of my builds.
I also smear some Three Bond TB-1211 on the center o-ring just to give it some extra sealing ability. Yea I know the factory manual doesn't tell you to do that, but I do!!!
None of my KZ motors ever had a head gasket leak over the years...........
1972 H2 750 Cafe Racer built in 1974.
1976 KH400 Production Road Racer.
1979 Kz1000 MK. II Old AMA/WERA Superbike.
1986 RG500G 2 stroke terror.
1986 GSXR750RG The one with the clutch that rattles!