Why is it der Oldenburger?

Oldenburger ends in -er, and most nouns ending in -er are masculine: in a German dictionary, 86% of them take der.

All nouns ending in -er
Sense 1
der

Meaning

[1] residents of Oldenburg, someone who comes from Oldenburg

[1] Bewohner von Oldenburg, jemand der aus Oldenburg stammt

Examples

[1] My neighbor is a native of Oldenburg

[1] Mein Wohnungsnachbar ist ein gebürtiger Oldenburger.

Declension

1 Singular Plural
Nominativ der Oldenburger die Oldenburger
Genitiv des Oldenburgers der Oldenburger
Dativ dem Oldenburger den Oldenburgern
Akkusativ den Oldenburger die Oldenburger

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary