In order to install Fnatic OP, go to: https://gear.fnatic.com/en/op/ and the link will lead you to the following website:
On the website, you can find OP software for MAC and Windows Generation 1 and 2 mice/keyboards.
If you would like to download MAC OP, scroll down and click on the following:
For Windows users, the download link is located here:
If you have Generation 1 Gear (Rush and Flick1) you can download the OP here: