I think this is probably because it's almost impossible to explain how to write omni. Other POVs are what they are because of rules. For example, third limited means you describe *only* what the POV character is aware of and share *only* their thoughts. Omni is, in a very real way, the lack of rules. The narrator can share whatever she wants, however she wants. This is what makes the POV so difficult because it's easy to share too much, or the wrong thing.Nope. Afraid not. There's very little on it, and no one really explains how to do it beyond examples. The only book that I've seen that is useful is Bob Mayer's The Writer's Toolkit. He has written in omni and discusses how to do it a bit, but even he doesn't quite sure of the workings.
Every writer also has to find their own way of doing omni, and probably rediscover this for each project. There's no right or wrong, really, and as I'm rewriting my Nano book into omni I'm realizing that I'm using very different techniques even from scene to scene within the same project. The book was originally in 3rd limited, and some scenes have to be rewritten from scratch because the omni narrator is seeing them so differently, often using almost an objective POV. But others undergo only minor changes, because the narrator is remaining focused on one character and the only changes are to match with the narrator's voice. I know that the omni books I've really enjoyed have often had this kind of variance, sometimes a true 'eye of a god' POV and sometimes very close to a certain character. Of course, this can change within the scope of a single scene as well, with the narrator going from objective view to being close to a character, and then even switching to follow other characters or returning to objective if the scene is a long one.