Do you have to be on NixOS to use the OCI images outputtable by nixpks/nix package manager or can you build OCI images using Nix from Mac/Windows/other non NixOS Linux?
They're just normal OCI images, so they can be run by any OCI-compatible container runtime. You can build images from any system running Nix, though you might encounter trouble on non-Linux systems because you may need to cross-compile the packages you use. The build process is exactly the same on any Linux distro (including NixOS and anything else) though.
No
> can you build OCI images using Nix from Mac/Windows/other non NixOS Linux?
Non NixOS Unix, not Windows yet, unless you use WSL or a VM