sea nymph vs Oceanid

sea nymph and Oceanid both are nouns.

Word	Noun	Adjective	Verb	Adverb
sea nymph	Yes	No	No	No
Oceanid	Yes	No	No	No

As nouns, Oceanid is a hyponym of sea nymph; that is, Oceanid is a word with a more specific, narrower meaning than sea nymph:

sea nymph: (Greek mythology) a water nymph who was the daughter of Oceanus or Nereus
Oceanid: (Greek mythology) sea nymph who was a daughter of Oceanus and Tethys

Other hyponyms of sea nymph include Calypso, Nereid, Siren.

sea nymph (noun)	Oceanid (noun)
(Greek mythology) a water nymph who was the daughter of Oceanus or Nereus	(Greek mythology) sea nymph who was a daughter of Oceanus and Tethys