The Collaborative International Dictionary of English v.0.48:

Oilstone \Oil"stone`\, n.
   A variety of hone slate, or whetstone, used for whetting
   tools when lubricated with oil.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Whet \Whet\, n.
   [1913 Webster]
   1. The act of whetting.
      [1913 Webster]

   2. That which whets or sharpens; esp., an appetizer. "Sips,
      drams, and whets." --Spectator.
      [1913 Webster]

   Whet slate (Min.), a variety of slate used for sharpening
      cutting instruments; novaculite; -- called also whetstone
      slate, and oilstone.
      [1913 Webster]

WordNet (r) 3.0 (2006):

oilstone
    n 1: a whetstone for use with oil