holder vs oarlock

holder and oarlock both are nouns.

Word	Noun	Adjective	Verb	Adverb
holder	Yes	No	No	No
oarlock	Yes	No	No	No

As nouns, oarlock is a hyponym of holder; that is, oarlock is a word with a more specific, narrower meaning than holder:

holder: a holding device
oarlock: a holder attached to the gunwale of a boat that holds the oar in place and acts as a fulcrum for rowing

Other hyponyms of holder include candle holder, candlestick, cigarette holder, keyboard, peg, pin, rowlock, thole, tholepin, zarf.

holder (noun)	oarlock (noun)
a holding device	a holder attached to the gunwale of a boat that holds the oar in place and acts as a fulcrum for rowing
the person who is in possession of a check or note or bond or document of title that is endorsed to him or to whoever holds it
a person who holds something