Found within the Fnatic mode settings (Settings > Utilities > Gaming mode), the Easter egg is activated by tapping on the Fnatic logo five times. Once you do that, a text box will appear wherein you have to enter the text "alwaysfnatic" without the quotes.
Once you do that, the Easter egg is unlocked with a Fnatic logo animation. Therein you will be presented with three new wallpapers with the Fnatic logo. You can choose to add these to your default wallpaper collection or just ignore them.
Personally, I'm not particularly fond of any of these but if you are and have a OnePlus device that includes the Fnatic mode then you can go ahead try the Easter egg out for yourself.