Osaka: a man drops his phone in his own beer. His friend says "アホやなあ" with enormous warmth, and both of them are delighted.
Tokyo: a man drops his phone in his own beer. His friend says "バカだなあ" with enormous warmth, and both of them are delighted.
Now swap the two sentences between the two cities and watch two friendships develop a small, unexplained crack.
Both words mean idiot. The difference is not meaning, it is temperature — and temperature is entirely regional.
In Kansai, アホ is the soft one. It is used constantly, affectionately, between friends and family, and it is the word Osaka comedy runs on. バカ, by contrast, sounds genuinely harsh: cold, judgmental, an actual insult.
In Kanto it is the other way round. バカ is the everyday, semi-affectionate one, and アホ carries a nastier edge — a bit contemptuous, a bit dismissive.
This is one of the clearest markers of the east–west divide in Japanese, and people here are extremely aware of it. Osaka people will happily tell you that being called アホ in Osaka means you are family, and that being called バカ means you should sit down and think about what you did.
Here is the useful bit. In Osaka, being called an idiot means you have been let in. Kansai measures closeness by how rude people are allowed to be to you — so an Osaka person who is unfailingly polite to you after three meetings is not being nice. They are keeping you outside. In Tokyo the signal runs the other way. Same country. Opposite thermometer.