Let us remember that alliteration is defined as the use of two words or more in close proximity that begin with the same consonant sound. If we have a quick look at this brilliant poem, we can see that the most important incidence of alliteration in "Ozymandias" comes at the end of the poem in the last three lines, which also conveys the poem's central message or theme through irony:
Nothing beside remains. Round the decay
Of that colossal wreck, boundless and bare
The lone and level sands stretch far away.
Hopefully you will have noted the three examples of alliteration in this quote, in "boundless and bare," "lone and level" and finally "sands stretch." What is key to realise is that the alliteration places added emphasis on the words that are used to describe what remains of the mighty empire of Ozymandias. All of the words that use alliteration convey the ruin and destruction of time, and how even the mightiest of civilisations will eventually be forgotten and decay into dust. Alliteration then, in this poem, is used to highlight the central message that the poet wishes to convey.