As of September 2016, OS X has been renamed to macOS (presumably to align with Apple's other operating systems iOS and watchOS). There have now been two operating systems released under this moniker (macOS 10.11 "Sierra" and macOS 10.12 "High Sierra").
Given that it's been over a year since then, would it be possible to rename os-x to macos, keeping the old tag as a synonym? This would be consistent with how Ask Different (a.k.a. Apple.SE) does it.
There was a previous meta topic called Tag synonym request: [mac-os] to [os-x] (or vice versa), and the consensus was to make os-x the parent tag because macOS 10.11 "Sierra" was not released yet. Now that two versions have been released with the name, I think that the reason for that is no longer valid.