How do you get the 2018 MSI LPL Oh My God?
It was sold in the League of Legends store. Riot's description in the client reads: “This icon was released in the store for purchase during the 2018 esports season.”
Is the 2018 MSI LPL Oh My God still available?
Probably not. Riot tied the 2018 MSI LPL Oh My God to 2018 (store purchase), and it does not normally bring back past event rewards or bundles.
