The Collaborative International Dictionary of English v.0.48:

Omniscient \Om*nis"cient\, a. [Omni- + L. sciens, -entis, p. pr.
   of scire to know: cf. F. omniscient. See Science.]
   Having universal knowledge; knowing all things; infinitely
   knowing or wise; as, the omniscient God. --
   Om*nis"cient*ly, adv.
   [1913 Webster]

         For what can scape the eye
         Of God all-seeing, or deceive his heart
         Omniscient?                              --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

omniscient
    adj 1: infinitely wise [syn: all-knowing, omniscient]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "omniscient":
   all-knowing, all-powerful, all-seeing, all-wise, almighty,
   apperceptive, appercipient, apprehending, apprehensive, aware,
   boundless, changeless, cognizant, comprehending, conscious,
   creating, creative, eternal, eternally the same, everlasting,
   glorious, good, hallowed, highest, holy, immortal, immutable,
   infinite, insightful, intelligent, just, knowing, knowledgeable,
   limitless, loving, luminous, majestic, making, merciful, mindful,
   numinous, omnipotent, omnipresent, one, perceptive, percipient,
   permanent, perpetual, perspicacious, prehensile, radiant, sacred,
   sagacious, sensible, shaping, shrewd, sovereign, supreme, timeless,
   ubiquitous, unbounded, unchanging, undefined, understanding,
   unlimited, wise