Re: [O] keybinding for inserting simple item, simple plain list

[Adam Porter]

org-meta-return (bound to M-RET by default) does this when you're in a plain
list.  If you're not in a plain list, it inserts a heading.  So just put the
point in a list and it should do what you want.

If you want a key to both create a plain list when you're not in one and add
another item when you are in one, that wouldn't be hard to do.