The Collaborative International Dictionary of English v.0.48:

Ossify \Os"si*fy\, v. t. [imp. & p. p. Ossified; p. pr. & vb.
   n. Ossifying.] [L. os, ossis, bone + -fy: cf. F. ossifier.
   See Osseous.]
   1. (Physiol.) To form into bone; to change from a soft animal
      substance into bone, as by the deposition of lime salts.
      [1913 Webster]

   2. (Fig.): To harden; as, to ossify the heart. --Ruskin.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Ossify \Os"si*fy\, v. i. (Physiol.)
   To become bone; to change from a soft tissue to a hard bony
   tissue.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ossify
    v 1: become bony; "The tissue ossified"
    2: make rigid and set into a conventional pattern; "rigidify the
       training schedule"; "ossified teaching methods"; "slogans
       petrify our thinking" [syn: rigidify, ossify, petrify]
    3: cause to become hard and bony; "The disease ossified the
       tissue"

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "ossify":
   anneal, brutalize, calcify, callous, case harden, cornify, firm,
   fossilize, harden, indurate, inure, lapidify, lithify, petrify,
   steel, temper, toughen, vitrify