double-decker vs omnibus

double-decker and omnibus both are nouns.

Word	Noun	Adjective	Verb	Adverb
double-decker	Yes	No	No	No
omnibus	Yes	Yes	No	No

As nouns, double-decker and omnibus are synonyms defined as:

double-decker and omnibus: a vehicle carrying many passengers; used for public transport

Other synonyms of double-decker include autobus, bus, charabanc, coach, jitney, motorbus, motorcoach, passenger vehicle.

double-decker (noun)	omnibus (noun)
a vehicle carrying many passengers; used for public transport	a vehicle carrying many passengers; used for public transport
	an anthology of articles on a related subject or an anthology of the works of a single author

double-decker (adjective)	omnibus (adjective)
	providing for many things at once