"Ozymandias" has a tone of ironic solemnity. The irony emerges from the juxtaposition of Ozymandias's inflated vision of his power and grandeur as ruler of a mighty kingdom and what survives of it today: a broken statue scattered on an empty desert. We are told that "nothing beside remains." The huge statue he had carved of himself to intimidate people is in pieces. No trace of his kingdom still exists beyond the shattered statue. No "works" survive to cause people to tremble and "despair."
Yet the irony runs deeper. The inscription on the broken statue reads
‘My name is Ozymandias, king of kings:
Look on my works, ye Mighty, and despair!'
The mighty should, in fact, look on Ozymandias's "works" and despair, not because of his fearsome power but because what they see shows he has no power left. His words are empty. The mighty might do well to remind themselves they too are not so powerful as they might believe. This could--and will--happen to them as well.
The poem's irony is somber. We are asked not so much to laugh at the foibles of humans, a common target of irony, but to solemnly consider our own mortality and the vanity of human endeavors, our own similarity to Ozymandias, whose weakness is in his pride and inflated ego. Words such as "shattered," "sunk," and "lifeless" convey sobering images of the mortality we all share no matter how great we think we are.
We see as well that the only remnant of the former civilization comes from an artist, the sculptor of the statue, who "well those passions read/which still survive" in the "sneer" and "wrinkled lip" of the long-dead tyrant. Art, even shattered and fragile, seemingly outlives tyranny.
The end of the poem is quiet and solemn. We feel the emptiness and can experience the lonely echoes of the setting in the alliterations ("boundless and bare," "lone and level") and slow cadence of the lines, slowed by line breaks and commas:
Round the decay
Of that colossal wreck, boundless and bare
The lone and level sands stretch far away