I would suggest that the inscription in "Ozymandias" represents the poem's thematic core, upon which everything else in it rests. Ultimately, the inscription in question reads, "My name is Ozymandias, King of Kings; / Look on my Works, ye Mighty, and despair!" This inscription, taken by itself, contains within it a striking and powerful sense of grandeur. It is telling, in terms of providing an impression of Ozymandias, both as an imposing ruler and personality. But for all of that, consider the scene in which this inscription is found, where the traveler is describing ruins in the desert. This is the tension that rests at the heart of the poem; the ruler's proclamation must be weighed against the reality, in which he and all of his achievements have been conquered by time. This, then, is the thematic message the poem ultimately expresses; it is about the transitory nature of the human condition, and how all things will eventually be swept away with time.