The Collaborative International Dictionary of English v.0.48:

Often \Of"ten\ ([o^]f"'n; 115), adv. [Compar. Oftener
   ([o^]f"'n*[~e]r); superl. Oftenest.] [Formerly also ofte,
   fr. oft. See Oft., adv.]
   Frequently; many times; not seldom.
   [1913 Webster]

WordNet (r) 3.0 (2006):

oftener
    adv 1: more often or more frequently