au fait vs up on

au fait and up on both are adjectives.

Word	Noun	Adjective	Verb	Adverb
au fait	No	Yes	No	No
up on	No	Yes	No	No

As adjectives, au fait and up on are synonyms defined as:

au fait and up on: being up to particular standard or level especially in being up to date in knowledge

Other synonyms of au fait include abreast, au courant.

au fait (adjective)	up on (adjective)
being up to particular standard or level especially in being up to date in knowledge	being up to particular standard or level especially in being up to date in knowledge