Source : Webster's Revised Unabridged Dictionary (1913)

Often \Of`ten\, adv. [Compar. {Oftener}; superl. {Oftenest}.]
   [Formerly also ofte, fr. oft. See {Oft}., adv.]
   Frequently; many times; not seldom.

Source : WordNet®

oftener
     adv : more often or more frequently