The Collaborative International Dictionary of English v.0.48:
onomancy \on"o*man`cy\, n. [Gr. 'o`noma name + -mancy. Cf. Nomancy.] Divination by the letters of a name; nomancy. [R.] --Camden. [1913 Webster] OnomanticWordNet (r) 3.0 (2006):
onomancy n 1: divination by the letters of a name