First of all, forget third person omnescient The designation is created by English teachers who didn’t write, and is nonsense.
As a matter of fact, it is extremely rare to actually see a third-person omniscient narration, mostly because it’s extremely difficult to do. I’ve written hundreds of stories (40+ published) and never used it.
The key concept for a working writer is “point of view.” The point of view character is the one whose thoughts are known to the audience. This is why there is no omniscient, since an omnicient character, by definition, would know what everyone around him is thinking. It might be useful for a telepath, but for no one else.
A better designation:
Third person, single point of view. The narration is in the 3rd person, but seen through the eyes of one character. This is by far the most common type of third-person narrative. Basically, though you write in the third person, you give the thoughts of one character. That character can guess what another is thinking, but the author cannot get into the head of the other.
third person, multiple point of view. The person whose thoughts you are describing changes from one scene to the next. Warning – changing POW within one scene is very difficult and is usually considered a sign of sloppy writing. So you write one scene from Jim’s point of view, then the next from Sally’s, then Jim’s, etc. You can have more than two characters doing this, but it’s difficult.
Third person, storyteller Rare, but it can be useful. This is where the author can jump in as he tells the story and can get the omniscient narrator effect.
Third person, camera eye Also rare. There is no point of view; we see the characters actions as though on screen, with no attempt to get into their thinking.