First, a quick review.
- In first person the writer is entirely inside one character's head and the story is written as though that character were narrating it. "I went to the store, and I saw Jim there with another woman."
- Third person limited is a step removed from first person, or maybe even just a half step. The character is not the narrator but everything is still told from that one character's point-of-view. This allows for things like description ("Mary's blue eyes flashed") not available in first person ("My blue eyes flashed"), but the writer still can only see things through one character's eyes and that character must be present for any action to take place, except when related in memory.
- In the third person omniscient POV, the writer can enter anyone's head and go anywhere, even without the characters. So, in a story about Hurricane Katrina, the writer can not only present various characters' perspectives, she can show us the devastation throughout New Orleans––even reaction around the nation and the world––while the characters remain stranded in the Ninth Ward.
Third person omniscient is both the hardest and the easiest POV for telling a story. It's very easy to go along writing a story and throwing in another POV whenever it's convenient to express some idea or provide a description a more limited POV wouldn't allow for. But third person done well, and for the right reasons, actually requires more thought and planning than any of the other POVs, including the oddball second person (you).
First you need to decide why omniscient works best. Is it the classic different takes on the same incident? Is it a more panoramic work spanning generations or continents? Is it a modest short story but one where you need to present several characters' thoughts that might go contrary to the dialogue?
Next you need to decide how many points-of-view you will use. Will it be just a few major characters, like the members of a family or a group of friends remembering a night they were all together? If the work spans generations or eras will the POV perhaps pass father to son or landowner to landowner? When dealing with multiple POVs you need to focus on characters of equal weight, otherwise you risk a confusing cacophony of voices.
How will you denote the changes in POV? Will each individual have a book, a chapter, a paragraph, a section of the story, or will they continually interact as they might in a short story depicting a conversation among friends?
Finally, if you are going to include POV shifts you need to find some kind of balance. In general each POV should get roughly equal time otherwise the differing POVs will feel like a device for convenience, or worse, a mistake. This goes for stories as well as novels. If John, Jane, and Mary are conversing at a party, you can't have us in John's head through most of the story and pop into Mary's head only once when she observes John acting a little tipsy.
The third omniscient can be a useful and innovative POV, but it can also be the most challenging. New writers may want to master the discipline of limited point-of-view before venturing into the omniscient. It's a bit like learning to walk before you run.