No where.
But really. Omni has taken careful measure to erase himself from history, the fact that the Gulag Plague also destroyed a lot of history doesn't help. The only real way to find out ANYTHING about Omni is to ask someone who was alive during his first assault. Moridin, and Apparatus, are two characters who could be asked. Other than that...really it would be difficult to find out anything, though studying him and his droids(even from afar) can provide clues.
But really. Omni has taken careful measure to erase himself from history, the fact that the Gulag Plague also destroyed a lot of history doesn't help. The only real way to find out ANYTHING about Omni is to ask someone who was alive during his first assault. Moridin, and Apparatus, are two characters who could be asked. Other than that...really it would be difficult to find out anything, though studying him and his droids(even from afar) can provide clues.