Both situational and dramatic irony are used to convey the poem's central purpose: that all human beings and all human achievements are temporal and fleeting. No matter how prideful we are or how powerful we actually are in life, because our lives are impermanent, so are our achievements and our power.

Situational irony is created when what we would expect to happen differs from what actually does happen. This statue of Ozymandias, or Pharaoh Rameses II, was fifty-seven feet tall, according to the Oxford English Dictionary. It was commissioned in order to display the pharaoh's power and might, yet now it lies shattered and forgotten in the sand, which is itself symbolic of the passage of time. We would not expect such a grand statue to be reduced to such detritus, and this is what constitutes the irony as well as shows how the poem conveys the idea that, in time, even the "greatest" of us can be forgotten.

Dramatic irony is created when the reader knows more than a character does. In this poem, if we think of Rameses II as a character, then we certainly know more than he does. He seems not to have realized that the sculptor who crafted the pharaoh's "sneer of cold command" was judgmental of rather than awestruck by him; further, Rameses II must have believed the inscription on the pedestal, not knowing or realizing that time would render him and his "Works" forgotten. He thought that his fame and stories of his might would live forever, but they have not. This also helps to convey the poem's meaning.

"Ozymandias" by Percy Bysshe Shelley is a poem told by an anonymous narrator who encounters a traveler who tells of a fallen and shattered statue in a remote area in the desert. The statue is of Ozymandias (also known as Ramesses II or Ramses the Great). The irony is situational.

The point of the statue is to emphasize the greatness of the Pharaoh and the way his works and his fame, like the stone of the statue, will endure forever. That expectation is reflected in the inscription: 

Look on my works, ye Mighty, and despair!

The pharaoh and his kingdom have both dwindled to the point that they are only remembered as curiosities by idle tourists and inspire thoughts about the nature of art and the skill of the sculptor rather than awe and despair in response to the pharaoh himself. In fact, the main lesson of the statue is that art outlasts its subject, but even so all humans and their creations are ephemeral in nature, subject to decay and corruption. 

The narrator of Ozymandias by Percy Bysshe Shelley is obviously well-traveled and interesting as he comes from "an antique land," suggesting that he is perhaps mysterious and exotic and seemingly knowledgeable. He   reflects on the scene before him and immediately recognizes the irony in what is left of "Ozymandias, king of kings..." in the form of his broken and derelict statue. Far from reflecting his grandeur, Ozymandias's statue is a testament to his pride and arrogance and it is ironic that even the sculptor could see it when he was working; to the point that the sculptor sculpted the " frown, And wrinkled lip, and sneer of cold command,..."

Further irony is present as, the king (Ramses II), does not even recognize it in the finished product; when the sculpture is completed. He is far too vain. The king thinks that the sculpture reveals his greatness to the point that other significant rulers whom he calls, "the Mighty," will "despair," as they cannot match his importance. Ozymandias is also convinced that he will be remembered in history as a great ruler whereas, in fact, he is exposed as a tyrant.

The fact that the "shattered visage," still exists but only to discredit him and lies apart from, "Two vast and trunkless legs of stone..." adds to the mocking tone of Shelley's poem. They, the stone blocks, are imposing but, without a head, they signify nothing and it is fitting that such a manipulative king should suffer this fate. The statue becomes offensive and, "a colossal wreck" suffering "decay." There is nothing majestic in these descriptions.

The words on the inscription intensify the insult because anyone who  looks "on my works," will be less than impressed with what surrounds the statue; noting more than, "The lone and level sands," which are "boundless and bare."  

The central irony in this poem is that Ozymandias's statue was intended to project his greatness. But when the onlooker sees it, it is not only shattered, but it lies in the midst of a wasteland. There is an inscription on the statue, which reads as follows:

My name is Ozymandias, king of kings; Look on my works, ye mighty, and despair!

This inscription, however, seems ludicrous in its modern surroundings. Rather than making the onlooker contemplate the majesty and power of the great Ozymandias, which was its intent, the inscription and indeed the entire scene leave one contemplating how short life is, and how time makes victims of us all. So, ironically Ozymandias's statue has exactly the opposite effect that the king intended.