first base vs opening move

first base and opening move both are nouns.

Word	Noun	Adjective	Verb	Adverb
first base	Yes	No	No	No
opening move	Yes	No	No	No

As nouns, opening move is a hypernym of first base; that is, opening move is a word with a broader meaning than first base:

Other hypernyms of first base include first step, initiative, opening.

first base (noun)	opening move (noun)
the initial stage in accomplishing something	the first of a series of actions
the fielding position of the player on a baseball team who is stationed at first of the bases in the infield (counting counterclockwise from home plate)
the base that must be touched first by a base runner in baseball