Adding VoodooPS2
For OS X 10.11 (El Capitan) and lower, you will instead need to download the original PS2 driver by RehabMan here. Note that these older drivers are built for x64 systems, but you can manually build the x32 versions in Xcode with the source code here.
Start by downloading the latest release version of VoodooPS2 from here (for macOS 10.12 Sierra and higher).
Add the kext to the Kexts folder in your OpenCore disk image, and remember to OC Snapshot the config.plist when finished. The necessary kexts (VoodooPS2Keyboard, VoodooPS2Mouse) will be added automatically for you by ProperTree.
Now, boot up macOS, and enjoy switching your keyboard and mouse from macOS to Linux!
Usually, the Option (⌥) and Command (⌘) keys on the keyboard maps to the Alt and the Windows (⊞) keys respectively. The original PS2 driver (by RehabMan) will swap these keymaps, however.
You can change this behavior is to go to Settings > Keyboard > Keyboard Shortcuts > Modifier Keys (or something equivalent) in macOS, and swap the action keys for Option (⌥) and Command (⌘).