Are oral antibiotics sufficient treatment for diverticulitis and diverticulitis?

Depends. In mild uncomplicated cases of diverticulitis, oral antibiotics are sufficient treatment for diverticulitis. More severe episodes may require hospital admission and intravenous antibiotics.
Sometimes. Diverticulitis occurs when one of the small diverticular "pouches" of the wall of the colon ruptures, leading to the egress of intestinal contents. If a neighboring structure or organ "band-aids" this hole closed, minor inflammation will follow, amenable to outpatient rx with oral antibiotics. If the hole does not seal rapidly, IV antibiotics or even surgery may be necessary.