Onision is a name Greg has been using online since 2005.
Origin and Meaning
In 2004, Greg began going by "OniSan" and "Unision" online. The following year, he began going by Onision. (Source) It seems he may have combined his two previous usernames to come up with Onision.
Throughout the early years of using "Onision" online, Greg gave several explanations on the meaning of the word. In 2005, he said: "You will not find this word anywhere but where I have placed it, it is me, it is original, it is exciting, odd, frightful, deep, honorable, harsh, mighty... strong ~ it is something no one can be, but he who created it... it is I, Gregory J... not one person can say a greater word to my ear... to the world it is worthless, to my world, it is all that truly matters, as long as my soul burns." (Source)
In 2008, in his FAQ, Greg states, "Onision" does not mean anything, it is a word I made up. So I suppose because it did not exist before me, "Onision" essentially is me." (Source)
When Greg began perusing a career on Youtube, he began talking about an "Onision Revolution" and began breaking down the word Onision as "Oni: One, Sion: Divine Community" (Source):
Greg continued to use the "one diving community" definition as late as 2011, but also began saying: "I just made it up. I typed what sounded good in my head, into Google, and at the time, nothing came up. So I went with it." (Source)
Greg created the Onision Youtube channel in 2006, but did not seriously start perusing a Youtube career until 2008. He later created several "Onision" branch off channels: OnisionSpeaks, OnisionArchive, and OnisionEncore. He uses various Onision URLs to host different iterations of the Onision Forums for his Youtube fans.
Since his Youtube career took off, Greg continues to go by Onision online on various social media.