At store.manacube.com the OG suffix will not be under the titles like a regular one would. It is meant to signify someone who has bought a rank between September 12th, 2013 - August 12th, 2016. You are unable to obtain this rank unless you have done so.
To get the suffix you must navigate to the link I have given you to the store (store.manacube.com) and on the side where it has categories such as "ranks," "black market," "titles," etc, you must scroll down until you find "claim packages" and click on it.
The first item should be the 'OG' title. If you click the "buy" button next to it will provide you with the neccesary requirements to obtain that title. (It is free because it's a gift to some of the early donators.)