Tokyo dialects

I was overhearing a conversation between two young ladies on a train and either they were using a lot of slang words or they were speaking dialect. I know it was japanese but the words were so odd.

Was thinking of osaaka-ben but it felt a bit odd as I was in Tokyo.

Is there any proper word for slang words in japanese?

Why would it be so strange that people speak Osaka ben in Tokyo? Tokyo is a big city and some people here have come from Osaka.

I agree with LikeBike. And there are at least three different dialects in Osaka alone, just as there are various dialects in New York or London.

「Is there any proper word for slang words in japanese?」
Not really, but that's the thing: slang is generally improper, in the sense that it usually either goes against normal grammar rules or the standard definition of the words.

As for hearing Osaka-ben in Tokyo, as LikeBike said, if two people who grew up in Osaka are talking to each other in Tokyo, they'll probably speak Kansai-ben. They could also have been speaking another dialect such as Tohoku, Hokkaido, or Okinawa dialect.