In many ways, the rules for writing in omniscient point of view are almost the exact opposite of those for writing in a closer perspective. In omniscient POV, the narrator isn’t stuck inside the protagonist’s perspective, but instead sees and knows everything. The omniscient narrator can tell the reader what happened five hundred years ago before the protagonist was born and what is happening inside the head of a random lady crossing the street in front of the protagonist’s car (that is, if it’s relevant to the story!)
The more distance you put between the narrator’s POV and the main character’s POV, the harder it is to write interior monologue without using thought tags. In omniscient point of view, the narrator might just need those thought tags to tell the reader what other characters are thinking. But not always, so do ask yourself if there is a better way each time you insert a thought tag!
In addition to the use of thought tags, there are a few other issues to keep in mind when writing in omniscient POV. The big one is the use of italics to indicate that something is going on in a character’s head. While it may be common in some genres, once again, don’t do it! Always remember that italics are hard on the eyes and can be hard to follow along with for more than a line or two. Even italicizing a line or two can be distracting for the reader.
Science fiction and fantasy are two genres in which you will find a lot of italicized text, not only in place of thought tags, but also to indicate a switch in narrative viewpoint. For example, a story might switch from a storyteller POV to that of a god or goddess who has a few words (or even an entire chapter!) of wisdom to impart.
While this may be typical in certain genres, that does not mean it can’t be done better in a different way. (Just because you can, doesn’t mean you should!) It is best to consider your reader’s needs in such a situation. Do you want some potential reader to flip through your book in a store, see large blocks of italicized text, and decide not to read your book simply because they don’t like the font?
Sometimes, despite your best effort, you may find that you really do need some sort of visual indicator to signal to the reader that something is changing in your story. An italicized font may be an easy visual indicator to use, but it is hardly the best one. Instead, consider using some other type of visual in these cases.
For example, let’s say your story contains several passages from the POV of a goddess who is informing the reader of some important history of the narrator’s religion. What type of visual could you use to indicate that a section will be the words of the goddess as passed down to the reader as opposed to writing a full chapter of italicized text?
How about simply using a different type of chapter header than what you use for your other chapters? Or, you could pretend that one of your characters “found” an ancient manuscript that was purportedly written by the goddess herself. You could include relevant pages from this manuscript within your story. If you are teaching the reader some piece of history that is important for them to know to follow the story, you could include diary entries or letters written by someone who lived through the historical event or period. For current events, you could format a page like a newspaper story with a good, old fashioned headline to signal to the reader that this page is different from the rest of the text.