der Offenbacher
Offenbach resident
A male resident of the city of Offenbach am Main.
Als gebürtiger Offenbacher kennt er die besten Ecken der Stadt.
As a native Offenbach resident, he knows the best parts of the city.

Offenbach resident
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Offenbach resident
A male resident of the city of Offenbach am Main.
Als gebürtiger Offenbacher kennt er die besten Ecken der Stadt.
As a native Offenbach resident, he knows the best parts of the city.

Names for residents are often formed in German by adding the ending '-er' to the city name. This pattern works for many cities, for example 'Berlin' → 'Berliner' (Berlin resident).
Mein Nachbar ist ein gebürtiger Hamburger.
My neighbor is a native Hamburg resident.
Words like 'Offenbacher' can be used as a noun for a person or as an adjective for a thing. As an adjective, it is capitalized and does not take a declension ending.
Ein Offenbacher hat das Turnier gewonnen.
An Offenbach resident won the tournament.
Whether used as a noun (person) or as an adjective (association/belonging), derivations from geographical names ending in '-er' are always capitalized.
Die Offenbacher Verkehrsbetriebe haben neue Busse.
The Offenbach transport company has new buses.