on the face of it vs ostensibly

on the face of it and ostensibly both are adverbs.

Word	Noun	Adjective	Verb	Adverb
on the face of it	No	No	No	Yes
ostensibly	No	No	No	Yes

As adverbs, on the face of it and ostensibly are synonyms defined as:

Other synonyms of on the face of it include apparently, seemingly.

on the face of it (adverb)	ostensibly (adverb)
from appearances alone	from appearances alone