I found this guide to writing in the third person omniscient and third person limited interesting and useful:
I write in third person limited, including multiple points-of-view.
The omniscient viewpoint is said to be dated, but one author who always uses it is John Irving. He flips between third person limited and third person omniscient, which can be delightful and also a bit distracting, as he’ll suddenly appear on your shoulder whispering in your ear that he knows something that’s going to happen to the protagonist before they do—or you as the reader do—and can you work out what I mean, you slowcoach?!