The Collaborative International Dictionary of English v.0.48:

Oillet \Oil"let\, n. [See Eyelet.] (Arch.)
   (a) A small opening or loophole, sometimes circular, used in
       mediaeval fortifications.
   (b) A small circular opening, and ring of moldings
       surrounding it, used in window tracery in Gothic
       architecture. [Written also oylet.]
       [1913 Webster]