The Collaborative International Dictionary of English v.0.48:

obsessed \obsessed\ adj.
   1. having or showing excessive or compulsive concern; -- used
      with with.

   Syn: haunted, preoccupied, taken up(predicate).
        [WordNet 1.5]

   2. influenced or controlled by a powerful force such as a
      strong emotion.

   Syn: possessed(predicate).
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

obsessed
    adj 1: having or showing excessive or compulsive concern with
           something; "became more and more haunted by the stupid
           riddle"; "was absolutely obsessed with the girl"; "got no
           help from his wife who was preoccupied with the
           children"; "he was taken up in worry for the old woman"
           [syn: haunted, obsessed, preoccupied, taken
           up(p)]
    2: influenced or controlled by a powerful force such as a strong
       emotion; "by love possessed" [syn: obsessed,
       possessed(p)]

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "obsessed":
   absorbed, absorbed in, affected, agonized, bedeviled, beset,
   besotted, bewitched, buried in, caught up in, contemplating,
   contemplative, devoted, devoted to, devoured by, dogged, dominated,
   engaged, engrossed, engrossed in, fixated, ghost-haunted,
   ghost-ridden, gripped, hag-ridden, harassed, haunted, held, hipped,
   hung-up, imbued with, immersed in, impressed, impressed with,
   infatuated, intent, intent on, involved, lost in, meditating,
   meditative, mindful, monomaniac, monomaniacal, monopolized, moved,
   nagged, obsessed by, occupied, overcome, penetrated with, plagued,
   possessed, preoccupied, prepossessed, queer, racked, rankled,
   remembering, seized with, single-minded, specter-haunted,
   spirit-haunted, spooked, spooky, stricken, studious, studying,
   submerged in, swept up, taken up with, torn, tortured,
   totally absorbed, touched, troubled, unable to forget,
   witch-charmed, witch-held, witch-struck, witched, wracked,
   wrapped in, wrapped up in



The Devil's Dictionary (1881-1906):

OBSESSED, p.p.  Vexed by an evil spirit, like the Gadarene swine and
other critics.  Obsession was once more common than it is now.
Arasthus tells of a peasant who was occupied by a different devil for
every day in the week, and on Sundays by two.  They were frequently
seen, always walking in his shadow, when he had one, but were finally
driven away by the village notary, a holy man; but they took the
peasant with them, for he vanished utterly.  A devil thrown out of a
woman by the Archbishop of Rheims ran through the trees, pursued by a
hundred persons, until the open country was reached, where by a leap
higher than a church spire he escaped into a bird.  A chaplain in
Cromwell's army exorcised a soldier's obsessing devil by throwing the
soldier into the water, when the devil came to the surface.  The
soldier, unfortunately, did not.