To say what others have answered in different words:
G-d's Holy Name, The Tetragrammaton, is a combination of “Haya” (was), “Hoveh” (is) and “Yihyeh” (will be).
When we say G-d exists out of time we're saying that for G-d the past, present, and future are as one. G-d does not experience time linearly, as we do. Rather, he sees it all as now. (See here)
In this sense, G-d is omniscient because all that will happen to us in the future is already known by G-d, since he - so to speak - is experiencing it now (or has already experienced it).
So, while the outcome of a certain action may presently be unknown to us (due to the Uncertainty Principle), in the future (once it has already occurred) we will know the outcome.
G-d knows that future now.
In response to the comment. This is what is says on Wikipedia (Which is just about the extent of my knowledge of the subject):
To measure the velocity of a particle, one must bounce other particles off of it, but such detection necessarily affects the particle being measured. The uncertainty principle says, for instance, that it is impossible to measure a particle's velocity in any moment and then have any hope of measuring its location for that moment (since the act of measurement of velocity immediately changed that particle's location). The observer must choose their knowledge of one time: the particle's location, or knowledge of its velocity.
To focus on the first part of that statement, this is my understanding of it. When observing something outside of yourself, you must interact with it. By definition, observing is something you do to something else, something separate from you. Since it is separate from you, the only way to examine it is interact with it. Interacting with it however, will affect it, and therefore we can never really know what it truly is, only how it reacts to our interaction.
But what if the thing you're examining is not separate from you, what if you just know it because it's you, not because you've examined something external?
To quote from this article (and please read it for context):
Maimonides, therefore, states that if we are to ascribe to G-d the knowledge of all beings and all events, we must conclude that: (a) His knowledge of the countless facts that comprise our existence are, in truth, but a single knowing -- His knowledge of self (since what we call "existence" is merely the expression of His infinite potential to create); and (b) He does not know Himself via a "mind" that is a distinct from Him, but that He, His knowledge and His "mind" are an utterly singular unit.
If that's the case, the Uncertainty Principle wouldn't kick in, since G-d wouldn't have to examine it in order to know it.