Alliteration is the repetition of the consonant sound at the beginning of consecutive words or words that are close together in the line of poetry. The first example of alliteration in "Ozymandias" by Percy Shelley is in lines 3 - 4 with the words "stone" and "stand".
"Who said: Two vast and trunkless legs of stone
Stand in the desert…. Near them, on the sand,"
A second example begins in line 4 and ends in line 5 with the words "sand" and "sunk".
"Stand in the desert…. Near them, on the sand,
Half sunk, a shattered visage lies, whose frown"
The third example is in line 8:
"The hand that mocked them, and the heart that fed"
Line 10 also contains an example -- "'My name is Ozymandias, king of kings".