This is a multi-center, parallel, active comparator controlled, open-label, randomized (1:1) phase III study of single agent ofatumumab compared to single agent rituximab in subjects with rituximab-sensitive indolent B-cell non hodgkin lymphoma that has relapsed at least 6 months after completing treatment with single agent rituximab or a rituximab-containing regimen. Subjects must have attained a Complete Response or Partial Response to their last prior rituximab containing therapy lasting at least six months beyond the end of rituximab therapy. Subjects will receive four weekly doses of single agent ofatumumab (1000 mg) or rituximab (375 mg/m2), followed by ofatumumab (1000 mg) or rituximab (375 mg/m2) every 2 months for four additional doses. Therefore, subjects will receive a total of eight doses of anti-CD20 antibody over 9 months. Subjects will be evaluated for response after completion of the first four doses of therapy, after six doses of therapy, and after completion of study therapy. Subjects will be followed until the end of the designated follow-up period (total study duration of 200 weeks) or until they meet the withdrawal criteria.