In order to install Fnatic OP, go to and the link will lead you to the following website:

When you scroll down, you can find OP software for MAC and Windows Generation 1 and 2 Mice/Keyboards. 

Just click on the Download button depending on the device you are using the Mice/Keyboard on:

If you have Generation 1 Gear (Rush and Flick1) you can download the OP here: