Omni-Man is a Viltrumite living on Earth. He is also known by his regular name: Nolan Grayson. As a way to make money, and to preserve his identity as Omni-Man, Nolan takes a job as an author for travel books.
When Nolan disappears from Earth after fighting Mark,
Cecil mentions that
"all his travel books always sold well, but sales will see a significant spike after his death."
This indicates that Nolan's death would drive up sales of his books even more, so he must be well known and pretty popular.
How is it that no one would recognise Nolan Grayson as the revered Omni-Man? Omni-Man didn't wear a mask, so it's incredibly easy to see his face. Was Nolan Grayson - The Author somehow different in appearance from Nolan Grayson - Omni-Man? Did no one ever make the connection?