Firstly I’d like to specify something: I’m not a dev, just a new user so take everything I say with an “IMHO” disclaimer.
That said, I don’t think it’s a matter of OS, rather of the keymap each workstation uses. I’ve had some hiccups because of this: all the machines here use Windows 10, but each user is free to choose his favourite keyboard (US, UK, DK, ES, SW, SE, IT, FI… we’ve got 'em all!)
Since OnlyKey works as a keyboard and the same keyboard spits out different symbols depending on the keymap used, I don’t think there is a fix.
The only workaround I can think of is using onlykey-cli to change the keymap on the fly (see docs), but other than that I think you’ll have to check the keymap before you use your onlykey.
Since I mostly use it to enter my credentials, I now just make sure that the symbols I use are in the same physical location on every keymap (that means only
.,! ), but if you need a specific symbol I think your only options are either change the keymap on the computer to match your OnlyKey or use the onlykey-cli to change the keymap on your OnlyKey.