Answer:
While the theme of "Ode on Grecian urn" focuses on how art is eternal, the theme of "Ozymandias" focuses on how things change.
Explanation:
Both the poems are from the romantic period but their themes are contrasting to each other. The former poem focuses on the eternity and the unchanging truth while the latter focuses on the constant changes which are taking place around us. There is nothing as unchanging and everything in this world is subjected to mortality.