Anti-Onisions, or Anti-Os, was once a broad term used to describe a people that partially or fully dedicates their YouTube channel, Facebook Fan-page, Tumblr blog, or other form of social media to speak negatively about Greg in any way. After 2019, the term took on a new meaning.
It is rumored Greg is the one who originally came up with the term Anti-Onision.
(This part might need to be moved. It's not about the term, it's about the first organized group against Greg's Youtube persona.)
It appears the first crop of "Anti-Onisions" began when Greg began uploading harsh opinion videos to YouTube in 2009 to his Onision Channel and later that year creating his Speaks channel. Although many of these videos were deleted by Greg, from what was saved by others, it seems he mostly made "militant" anti-meat-eater and anti-religion videos. It seems Greg made enemies of the YouTube ranting community of the time. Many would make reply videos, speaking against points he made in his videos or even criticizing the execution of his opinions.
Greg added fuel to the fire when reply videos made by other ranters began disappearing. It turns out Greg had been filing False DMCA Claims against these channels, which is against YouTube's TOS. (Source)
The summer of 2010, a group of ranters formed W.O.R., which stood for World Onision Resistance or World Onision Rebellion. It seemed to be a channel collaborative effort to collect ranters videos deconstructing and criticizing Greg's harsh Speaks videos. The founder of the group eventually abandoned and deleted WOR due to members making up lies in order to make Greg look bad and to stop the spreading of Greg's home address. (Source) In September, it seems a couple of ranters attempted to revive the WOR movement by creating a new WOR channel, but it was abandoned in less than a month. (Source)
When Greg's channels began to take a dramatic turn after divorcing Skye in late 2010, many Anti-O pages and channels began popping up to follow the drama or to speak out against Greg. Since then, a steady number of antis seem to always be coming and going.
At the time, Greg very rarely uses the term "Anti-Onision". For many years, Greg referred to Anti-Os as "haters". For a brief time he called them "Smurfs". In 2015, he began referring to them as "Social Justice Warriors", although the term is usually used on the internet to describe people that preach about Social Justice Issues online, which is something he did often at the time.
After the chaos of the 2019 Accusations and Chris Hansen's involvement, there was an intense backlash facilitated by some Youtube commentary channels. They highlighted wrongdoings by anti-Onision figures such as Shiloh and Hansen. "Anti-O" became a term for people who take their hatred against Greg to extremes such as making false reports to police or harassing people who question the accusations against Greg.
Prominent figures who used to be referred to as "anti-onision" now back away from the term and deny being "anti-o" due to the immensely negative connotation with the term.
Greg now uses the term "anti-o" extremely often to describe anyone who is claiming he groomed minors or that he knowingly rated underage fans from his forums. He often uses examples of extreme actions against him by people online to support his claim that all anti-o's are crazy morons who are following the mob mentality against him.