Otaku slang for a deeply and broadly perverse individual, who enjoys a sexual response to essentially all sexual stimuli, fetishes, complexes, etc. From the latin "omni" meaning "all" and "con," a Japanese corruption of "complex."
"He's more than a lolicon, he's a full-on omnicon. He'll fap to anything."
Someone who can go from being a nice person into a deceptive opportunist with in nano seconds. One minute they are your best friend, the next they are walking off with the copyright draft of your first novel.
You never know someone is an omnicon beause they are deceptivley friendly and decisive in their plans. An omnicon could very well be classed as a visionary but due to their criminally minded behaviour and outlook, they are never referred to by anything less than opportunist.
How did your house painting go?
well it didnt really, the tradesman turned out to be an "omnicon", i came home to find to find the place emptied out.