divination vs onomancy

divination and onomancy both are nouns.

Word	Noun	Adjective	Verb	Adverb
divination	Yes	No	No	No
onomancy	Yes	No	No	No

As nouns, onomancy is a hyponym of divination; that is, onomancy is a word with a more specific, narrower meaning than divination:

divination: the art or gift of prophecy (or the pretense of prophecy) by supernatural means
onomancy: divination by the letters of a name

divination (noun)	onomancy (noun)
the art or gift of prophecy (or the pretense of prophecy) by supernatural means	divination by the letters of a name
a prediction uttered under divine inspiration
successful conjecture by unusual insight or good luck