Why is it das Ohio?

Ohio ends in -o, but that ending doesn’t settle the article: in a German dictionary, 45% of -o nouns take das, 43% take der, and 11% take die. So learn it with its article: das Ohio.

Which endings you can rely on
Sense 1
das

Meaning

[1] State in the northeast of the United States

[1] Bundesstaat im Nordosten der USA

Examples

[1] Cincinnati is located in Ohioe

[1] Cincinnati liegt in Ohio.

[1] The Ohio of the nineteenth century is particularly remembered because of the Toledo War

[1] Das Ohio des neunzehnten Jahrhunderts ist vor allem wegen des Toledo-Kriegs in Erinnerung geblieben.
Grammar note

The article is needed if "Ohio" in a certain quality, at a certain time or period as a subject or object in the sentence, is otherwise, i.e. usually, no article is used.

Declension

1 Singular Plural
Nominativ (das) Ohio —
Genitiv (des Ohio) (des Ohios) Ohios —
Dativ (dem) Ohio —
Akkusativ (das) Ohio —

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary