Anti-Onisions, or Anti-Os, is 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.
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.
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 revided 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.