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Webster 1913 Edition

Noun.

[Cf. F.

ontologiste

One who is versed in or treats of ontology.

Edin. Rev.

Noun.

One who treats of or considers the nature and qualities of being in general.

ontologist ‎(plural ontologists)

(philosophy) A scholar or theorist of ontology, or an adherent of a particular ontological doctrine.
- 2008 August 12, Luca Moretti, “The ontological status of minimal entities”, in Philosophical Studies, volume 141, number 1, DOI:10.1007/s11098-008-9265-6:
  On the other hand, nominalists and parsimonious ontologists who do not believe in the existence of properties, facts and abstract entities do stick to these colloquial usages of ‘property’ and ‘fact’ grounded in principles as platitudinous as [i]–[iii] without apparent problems.

scholar or theorist of ontology