Opi

From Opa +‎ -i.

Opi m (strong, genitive Opis, plural Opis, diminutive Opilein n)

The word is commonly used with a definite article in most parts of central Germany, southern Germany, and Austria: Wo ist der Opi? – "Where is Grandpa?" In northern Germany—and generally in writing—no article tends to be used: Wo ist Opi?
The genitive Opis is used before the modified term, without any article or determiner: Opis Tasche – “Grandpa’s bag”. This is chiefly northern German usage; the rest of the language prefers: die Tasche vom Opi (as above). — The genitive can also be used with an accompanying article or determiner: die Tasche meines Opis – “my grandpa’s bag”. This, however, is not common usage in any region; children will say instead: die Tasche von meinem Opi.

German