in Osaka dialect...

How can I say ''本当は行きたかったんじゃないの？”in Osaka dialect?
And does "...だと思う”become "やと思う”?

Yes, "だと思う" = "やと思う".

You can say "本当は行きたかったんじゃないの？”as
"本当は行きたかったんとちゃう？”in Osaka dialect.

ありがとう＾-＾
Does that mean the (じゃない）in the negative questions always becomes "..とちゃう"?

That's right.

I won't use 本当　but use ほんま instead :p

I thought it was strange because I already knew about (本当＞＞ほんま）
ありがとう（大阪弁でおおきにやろ？）

本当, ほんま, ほんと are all used regularly in Osaka. Not many people really say おおきに anymore in real life.

I see ^-^;
thank you.