The Collaborative International Dictionary of English v.0.48:

Orison \Or"i*son\, n. [OF. orison, oreson, oreison, F. oraison,
   fr. L. oratio speech, prayer. See Oration.]
   A prayer; a supplication. [Poetic] --Chaucer. Shak.
   [1913 Webster]

         Lowly they bowed, adoring, and began
         Their orisons, each morning duly paid.   --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

orison
    n 1: reverent petition to a deity [syn: prayer, petition,
         orison]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "orison":
   Angelus, Ave, Ave Maria, Hail Mary, Kyrie Eleison, Paternoster,
   aid prayer, appeal, application, beadroll, beads, beseechment,
   bidding prayer, breviary, chaplet, collect, communion,
   contemplation, devotions, entreaty, grace, impetration,
   imploration, imprecation, intercession, invocation, litany,
   meditation, obsecration, obtestation, petition, plea, prayer,
   prayer wheel, rogation, rosary, silent prayer, suit, supplication,
   thanks, thanksgiving