I have 7 threaded barrels for 9/40/45, and not a single one of them has an o-ring. They’re all standard threads. The metric threaded barrels often use o-rings because they’re designed to.
I do have 22LR thread protectors that have o-rings built into the caps, but that’s because they’re made to SiCo’s o-ring engagement specs.
Viton size 013 is what is used with a lot of 1/2-28 pistons/silencers. That might work if you can’t figure out how to tighten it so that it doesn’t come loose.
Alternatively, 1-2 wraps of teflon tape work. That’s what I use on my direct thread silencers.