Nels was one of my favorite characters on Little House on the Prairie. His nasty and spoiled daughter Nellie always made Laura’s life difficult.
I’ve seen Richard Bull in a few other shows. He was in a few episode of Highway to Heaven.
He was 89 years old.
R.I.P.
Nels was always one of my favorite characters in the show, too. I loved the scenes with him and Harriet especially. It’s my understanding that the cast is really close (from reading Alison Arngrim’s book and also from an opportunity I had to interview her), so I’m sure this will be very difficult for them.
In Alison Arngrim’s memoir (which I recommend enthusiastically as not just a great former-child-star memoir but as a great memoir in general) she says that of all the actors on the show he was by far the most like his character: very kind, dignified, slightly reserved, and intelligent.
Man, I immediately thought of Richard Moll, of Night Court fame.
I was sad to see this too. Another piece of my childhood gone. Nels was always one of my favorite characters on LHotP. 
First the Professor, now Mr. Oleson. sigh