“Ode on Melancholy,” as with Keats’s other odes, is a poem rich in sound and texture. There are a number of examples of alliteration throughout. The first example is in the opening line:
No, no, go not to Lethe, neither twist
The repeated /n/ sounds emphasize the way that this first stanza is about things that the melancholic individual should not do. The /n/ sounds are almost like the sound of an angry parent; they also have a slightly desperate sound, hinting at the tempting qualities of those intoxicants listed during the rest of the stanza. This link between the /n/ sound and the speaker’s admonishments is echoed in the “not” and “nor” found in lines 5 to 7 as well.
The last two lines of the stanza also use alliteration effectively. The phrase “shade to shade”—also an example of diacope—emphasizes the metaphorical darkness of giving in to death. The speaker thinks melancholy should be embraced, rather than dulled by intoxication or annihilated by suicide. These two "shade[s]" represent an excess of this darkness (contrasted with the preferred "wakeful anguish of the soul"). The two /dr/ sounds in "drowsily" and "drown" (lines 9 and 10) draw this same link too.
The next alliteration is found in line 11:
But when the melancholy fit shall fall
Sudden from heaven like a weeping cloud,
That fosters the droop-headed flowers all
The two /f/ sounds contribute to the line's shifts in tone, with the first /f/ helping give the word a sprightly and upward ring which in turn emphasizes the more imposing, downtrodden sound of "fall." The same sound is echoed in line 13, in which melancholy is compared to a "weeping cloud" that nurtures "flowers." This link between "foster[ing]" and "flowers" supports the more general relationship between melancholy and beauty.
Skipping ahead, lines 26 to 29 use a number of alliterating /s/ sounds (also known as sibilance). These come as the poem is building to its peak, describing a kind of climax of pleasure/enjoyment of beauty which, at the same time, represents an intense moment of melancholy:
Veil'd Melancholy has her sovran shrine
Though seen of none save him whose strenuous tongue
Can burst Joy's grape against his palate fine;
His soul shalt taste the sadness of her might,
These lines also focus on the act of tasting. The /s/ sounds have a kind of mouthwatering quality that helps bring out this focus on taste, capturing the way "Joy's grape" feels as its bursts in the mouth.