oral cavity vs oral fissure

oral cavity and oral fissure both are nouns.

Word	Noun	Adjective	Verb	Adverb
oral cavity	Yes	No	No	No
oral fissure	Yes	No	No	No

As nouns, oral cavity and oral fissure are synonyms defined as:

oral cavity and oral fissure: the opening through which food is taken in and vocalizations emerge

Other synonyms of oral cavity include mouth, rima oris.

oral cavity (noun)	oral fissure (noun)
the opening through which food is taken in and vocalizations emerge	the opening through which food is taken in and vocalizations emerge