The Collaborative International Dictionary of English v.0.48:

Oftensith \Of"ten*sith\ ([o^]f"'n*s[i^]th`), adv. [Often + sith
   time.]
   Frequently; often. [Obs.]
   [1913 Webster]

         For whom I sighed have so oftensith.     --Gascoigne.
   [1913 Webster]