The easiest way to express the ontological argument is like this (where "God" = a maximally great being, or a being that is omnipotent, omniscient, and morally perfect in every possible world):
1. God is either necessary or impossible. (Definition)
2. God is not impossible. (Premise)
3. Therefore, God is necessary. (From 1 and 2)
Transpositionally, (2) implies that God is possible. Given God's possible existence and S5, it follows that God exists. I argued earlier that one may demonstrate (2) by way of reductio ad absurdum. However, after giving it some further thought, I also think there is promise in arguing by way of analogy.
Take the proposition p1, "some of the existing apples are on earth." Now, according to Boolean logic, the quantifier, "some," does not necessarily imply that there are some existing apples that are not on earth. Nevertheless, given the possibility that p1, it is also reasonable to infer the possibility of p2, "all of the existing apples are X." In other words, the possibility of some implies the possibility, even if not the actuality, of all.
Applied to the ontological argument, we know that there are agents (such as ourselves) who possess some power, some knowledge, and some moral goodness in some possible worlds. If the some-all analogy is correct and taken to its logical conclusion, it follows that God possibly exists. This is all that is needed to show that (2) is true. Of course, (2) in conjunction with (1) implies that God is necessary, or has existence in all possible worlds.
Given that God exists in all possible worlds, and given that the actual world is a member of the set of possible worlds, it follows that God exists in the actual world. Therefore, God exists.
Of course, if one is not persuaded by the some-all analogy, then he/she will not necessarily accept the conclusion that God exists. For the rest of us, though, the knowledge of our limited perfections only confirm our conviction that God exists. As C.S. Lewis quips (and I paraphrase), we only know that a line is crooked if we have some idea of what a straight line looks like.