Hi Gary,
2018 5.3.4 states "Category 3 systems shall comply with 5.1.4".
As 5.1.4 describes all required valves in a Category 1 system, all valves in a Cat 1 system are required in a Cat 3 (and Cat 2).
With this in mind, a ZV for O2 would be required.
Is this helpful?
Bob