omnibus vs public transport

omnibus and public transport both are nouns.

Word	Noun	Adjective	Verb	Adverb
omnibus	Yes	Yes	No	No
public transport	Yes	No	No	No

As nouns, public transport is a hypernym of omnibus; that is, public transport is a word with a broader meaning than omnibus:

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

omnibus (adjective)	public transport (adjective)
providing for many things at once