onomastics vs lexicology

onomastics and lexicology both are nouns.

Word	Noun	Adjective	Verb	Adverb
onomastics	Yes	No	No	No
lexicology	Yes	No	No	No

As nouns, lexicology is a hypernym of onomastics; that is, lexicology is a word with a broader meaning than onomastics:

onomastics: the branch of lexicology that studies the forms and origins of proper names
lexicology: the branch of linguistics that studies the lexical component of language

onomastics (noun)	lexicology (noun)
the branch of lexicology that studies the forms and origins of proper names	the branch of linguistics that studies the lexical component of language