witness vs observer

witness and observer both are nouns.

Word	Noun	Adjective	Verb	Adverb
witness	Yes	No	Yes	No
observer	Yes	No	No	No

As nouns, observer is a hypernym of witness; that is, observer is a word with a broader meaning than witness:

witness: someone who sees an event and reports what happened
observer: a person who becomes aware (of things or events) through the senses

Other hypernyms of witness include beholder, perceiver, percipient, speaker, talker, utterer, verbaliser, verbalizer.

As nouns, observer is a hypernym of witness; that is, observer is a word with a broader meaning than witness:

witness: a close observer; someone who looks at something (such as an exhibition of some kind)
observer: a person who becomes aware (of things or events) through the senses

Other hypernyms of witness include beholder, perceiver, percipient.

witness (noun)	observer (noun)
testimony by word or deed to your religious faith	a person who becomes aware (of things or events) through the senses
someone who sees an event and reports what happened	an expert who observes and comments on something
a close observer; someone who looks at something (such as an exhibition of some kind)
(law) a person who attests to the genuineness of a document or signature by adding their own signature
(law) a person who testifies under oath in a court of law

witness (verb)	observer (verb)
be a witness to
perceive or be contemporaneous with