organism vs living thing

organism and living thing both are nouns.

Word	Noun	Adjective	Verb	Adverb
organism	Yes	No	No	No
living thing	Yes	No	No	No

As nouns, living thing is a hypernym of organism; that is, living thing is a word with a broader meaning than organism:

organism: a living thing that has (or can develop) the ability to act or function independently
living thing: a living (or once living) entity

Other hypernyms of organism include animate thing, causal agency, causal agent, cause.

organism (noun)	living thing (noun)
a living thing that has (or can develop) the ability to act or function independently	a living (or once living) entity
a system considered analogous in structure or function to a living body