Remap FN key to OPTION key

One Excellent third party tool that can make amazing edits to the keyboard mapping is named Karabiner.

To accomplish option and function key changes, create a filter for the external keyboard and adjust the modifier keys.

Prerequisites: external keyboard

1. Download, install and open Karabiner
2. Open in the menulet Karabiner -> Preferences
3. Open the tab Misc & Uninstall
4. Hit the button Launch EventViewer
5. Hit the tab Devices

6. Make a note of the vendor and product ID of the external keyboard:

   

7. Hit the tab Main

8. Enter the (PC-)Ctrl-key, (PC-)Fn-key and the (PC-)Alt-key and make a note of the "event names/miscs":

   

9. Close the EventViewer

10. Hit the button Open private.xml
11. Open the file private.xml with an appropriate editor

12. Enter the customized keycode and replace the vendor name, the product name, the vendor and product ID by IDs and names found above as well as the "names/miscs" of the events (= KeyCode).

    <?xml version="1.0"?>
    <root>
      <devicevendordef>
        <vendorname>Cougar</vendorname>
        <vendorid>0x05ac</vendorid>
      </devicevendordef>
      <deviceproductdef>
        <productname>ATTACKX3</productname>
        <productid>0x0256</productid>
      </deviceproductdef>
      <item>
        <name>Function to Option for ATTACKX3</name>
        <identifier>private.fn2optExtK</identifier>
        <device_only>DeviceVendor::Cougar, DeviceProduct::ATTACKX3</device_only>
        <autogen>__KeyToKey__ KeyCode::Fn, KeyCode::OPTION_L</autogen>
      </item>
    </root>
    

    Depending on your findings in the Event Viewer you may have to remap (PC-)keys to Control_L and Command_L also.

13. Save the file

14. Open in the menulet Karabiner -> Preferences the tab Change Key
15. Hit the ReloadXML button
16. Enable the remapping. It should be listed at the beginning.