In Japanese language, Oyaji  is an informal word meaning "dad".  "Gaggu" is the Japanese word for 'gag'.  Put them together and you have what is roughly equivalent to 'old man's joke'.  Another similar word is Dajere (駄洒落) which according to Wikipedia, is a play on words. The common response to an Oyaji Gaggu is "samui!" (literally means 'cold', but basically means "wow, really bad joke!")....Of course foreigners living in Japan think we are quite clever when we can show off our Japanese language skills by learning a few of these...Here are some of my favorites:

海豚　いるか　どうか　わからない
(iruka iru ka douka wakaranai)

Is the dolphin here or not?

'iruka' = dophin
'iruka douka wakaranai' = is here or not?
The gag: repeating 'iruka'

いくら　が　いくら　ですか？
(ikura ga ikura desu ka?)

How much does the ikura cost?

'ikura' = a popular sushi dish consisting of salmon roe. It also means 'how much is it'
The gag: repeating 'ikura'

はまち　は　How Muchy ですか？
(Hamachi wa how muchy desu ka?)

How much does the hamachi cost?

'hamachi' = another popular sushi dish consisting of yellowtail fish.
This one is a hybrid O.G...Mixing words in English and Japanese which sound the same.

楽すきかった
(tanosukikatta)

Enjoyed (skiing) - not real Japanese word! My invention...

"tanoshii" = to enjoy
"ski" = ski (English)
The gag: blending the two words into one.

Tomorrow 止まる？
(Tomorrow tomaru?)

Tomorrow (will you) stay overnight?

"tomaru" = stays overnight (like a friends house or hotel).
The gag: Mixing English and Japanese words that sound almost the same.

これはナンですか？
(kore wa nan desu ka?)

Is that Nan (Indian Bread)

The gag: 'kore wa nan desu ka' means 'what is that'.  The word for Indian bread is also nan...Get it?? Use this at an Indian restaurant...Waiters love it.