der Opi
grandpa
A colloquial and affectionate term for one's own grandfather.
Am Sonntag gehen wir immer bei Omi und Opi zum Mittagessen.
On Sundays, we always go to Grandma and Grandpa's for lunch.

grandpa / old man
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grandpa
A colloquial and affectionate term for one's own grandfather.
Am Sonntag gehen wir immer bei Omi und Opi zum Mittagessen.
On Sundays, we always go to Grandma and Grandpa's for lunch.

'Opi' (grandpa) is a term of endearment, an affectionate short form of 'Opa' (grandpa), which in turn is a short form of 'Großvater' (grandfather). The ending '-i' is often used in German for cute or family-related terms.
Als Kind habe ich meinen Opi jedes Wochenende besucht.
As a child, I visited my grandpa every weekend.
This word clearly belongs to colloquial language. It is very informal and is used mainly in family or private settings.
„Wo ist Opi?“, fragte das kleine Kind.
'Where is Grandpa?' asked the little child.
'Opi' (grandpa) forms a pair with 'die Omi' (grandma), the affectionate form for grandmother. Both words are very often used together.
Meine Omi und mein Opi sind seit über 50 Jahren verheiratet.
My grandma and my grandpa have been married for over 50 years.
When 'Opi' is used for an unfamiliar older man, it can sound neutral, but also slightly disrespectful. Tone of voice and the situation determine whether it is perceived as friendly or condescending.
Da vorne sitzt ein Opi auf der Bank und füttert die Tauben.
There's an old man sitting on the bench up ahead feeding the pigeons.
grandpa
'Opa' and 'Opi' are very similar colloquial words. Both can mean your own grandfather, and both can also be used informally for an older man. However, 'Opi' usually sounds even more affectionate, smaller, or more childlike than 'Opa'. Children often say 'Opi'; adults use it more in very familiar situations or jokingly. It is easy to confuse the words because they mean almost the same thing. As a safe choice in everyday language, 'Opa' is the more neutral familiar option; 'Opi' fits especially when it is meant to sound very warm or cute.

Der kleine Junge malt seinem Opi ein Bild zum Geburtstag.
The little boy paints his grandpa a picture for his birthday.
Im Gespräch mit einem Freund hat Frau Schmidt von ihrem Opa erzählt.
In a conversation with a friend, Frau Schmidt talked about her grandpa.