"Ozymandias" is obsessed with transience; the very fact that the statue is a "colossal wreck" (13) says loudly and clearly that some things just don't last forever. But the poem isn't just about how really big statues eventually succumb to the ravages of time; the statue is a symbol of Ozymandias's ambition, pride, and absolute power, and thus the poem also implies that kingdoms and political regimes will eventually crumble, leaving no trace of their existence except, perhaps, pathetic statues that no longer even have torsos.
Even though the poem is obsessed with transience and impermanence, it also suggests that a work of art, however fragmentary, leaves a record of what has passed away.
In the inscription on the pedestal Ozymandias calls himself the "king of kings" while also implying that his "works" – works of art like the statue, pyramids, that sort of thing – are the best around (10). Ozymandias thinks pretty highly of himself and of what he's achieved, both politically and artistically. The fact that he commissions this "colossal" statue with "vast legs" points to his sense of pride, while the statue's fragmentary state indicates the emptiness (at least in the long term) of Ozymandias's boast.
Ozymandias's proud statement that he is the "king of kings" aligns him with a number of power-hungry villains, like the Biblical Satan, or even Sauron from Lord of the Rings.
"Ozymandias" was inspired by a statue, and it's no surprise that art is one of this poem's themes. The traveler makes a point of telling us that the statue was made by a really skilled sculptor, and the poem as a whole explores the question of art's longevity. The statue is in part a stand-in or substitute for all kinds of art (painting, poetry, etc.), and the poem asks us to think not just about sculpture, but about the fate of other arts as well.
The sculptor is similar to the reader of the poem because both engage in the processes of reading and interpretation.
"Ozymandias" describes a statue, and statues are made from rocks and stones found in nature. While the poem explores the way in which art necessarily involves some kind of engagement with the natural world, it also thinks about how nature might fight back. The statue's head is half-buried in the sand, after all, and we are left wondering what role the erosive force of dust storms, wind, and rain played in its destruction.
"Ozymandias" suggests that the relationship between art and nature is a double-edged sword: while the natural world furnishes the artist with raw materials, it also has the power to reclaim those materials by later destroying the work of art.