When Lisa and Oliver moved to Hooterville, the other characters were on a first-name basis with each other, but the Douglases were always on a "Mr" and "Mrs" basis with everybody.

I can understand that maybe when you're just meeting everybody this may be the case, but you'd figure after living there for a while and getting to know the people, they'd be less formal and start being on a first-name basis?

After so many years they're still calling each other "Mr" and "Mrs"?

Same kind of thing happened on Bewitched with the Kravitzes.

I guess this was the '60s for you!

I've wondered that, though everyone called Mr Haney, Mr Haney

On Our Miss Brooks, Connie Brooks LIVED with Margaret Davis and still always addressed her as "Mrs. Davis."

Also look at The Beverly Hillbillies, Jed Clampett and Milburn Drysdale were never on a first-name basis, even after 9 seasons...!

The Douglases probably adopted that custom when they lived among the New York elites where everyone was formal, and just like their furniture, clothes, etc. they brought it right along with them to Hooterville without a second thought. This goes to show the ongoing cultural divide between Oliver and the townsfolk and his inability to adapt to the norms of his new community (a major theme of the show).

Lisa adapted extremely well by comparison, but for some reason she too insisted on the more formal way of addressing people. Maybe she was just following Oliver's lead and reciprocated when others called her Mrs. Douglas.