old-hat vs commonplace

old-hat is not a noun while commonplace is a noun.

Word	Noun	Adjective	Verb	Adverb
old-hat	No	Yes	No	No
commonplace	Yes	Yes	No	No

As adjectives, old-hat and commonplace are synonyms defined as:

old-hat and commonplace: repeated too often; overfamiliar through overuse

Other synonyms of old-hat include banal, hackneyed, shopworn, stock, threadbare, timeworn, tired, trite, well-worn.

old-hat (noun)	commonplace (noun)
	a trite or obvious remark

old-hat (adjective)	commonplace (adjective)
out of fashion	not challenging; dull and lacking excitement
repeated too often; overfamiliar through overuse	repeated too often; overfamiliar through overuse
	completely ordinary and unremarkable