oral exam vs viva

oral exam and viva both are nouns.

Word	Noun	Adjective	Verb	Adverb
oral exam	Yes	No	No	No
viva	Yes	No	No	No

As nouns, oral exam and viva are synonyms defined as:

Other synonyms of oral exam include oral, oral examination, viva voce.

oral exam (noun)	viva (noun)
an examination conducted by spoken communication	an examination conducted by spoken communication