The repeated use of the o sound in a host golden deffodils is called?

The repeated use of the "o" sound in "a host of golden daffodils" is called assonance.

Assonance is a literary device where vowel sounds are repeated in close proximity to create a musical effect. In this case, the repeated "o" sound in "host," "golden," and "daffodils" creates a sense of richness and fullness, reflecting the beauty of the daffodils.