Omniscient means, literally, knowing everything.
In terms of writing, it's the difference between a narrator who knows all and tells what he wants, and a narrator who only knows what the characters experience and think.
In Omnisicent, you could have (for example) a camera-like spiraling-in of viewpoint, starting with a bird in the sky and swooping across a plain, only to end up in a bedroom window, all the while sharing information about the storyworld. See King/Straub's Black House for an intro that does this.
In 3p limited, unless the character was doing the swooping and spying, that tool is out of the box.
In practical terms, since no one says "You must write this piece in 3pL" or "You must write this piece in Omni" (outside of markets or contests that require it, of course), it doesn't matter. If it suits your needs to step outside the heads of the characters, then do so--just be consistent about it, and be artful.
And don't bore the reader.