The present active participle of eimi (to be) — nominative masculine singular. Ōn means being, the one who is, existing. Theologically crucial in Revelation 1:4, 8 where it forms part of the divine self-description: 'who is (ho ōn) and who was and who is to come.'
The participial form ho ōn — 'the one who IS' — is the NT Greek rendering of the divine name revealed in Exodus 3:14: 'I AM WHO I AM' (ehyeh asher ehyeh). The LXX renders this as ego eimi ho ōn — 'I am the being one, the ever-existing.' Revelation 1:4 weaves this into the trinitarian greeting with startling grammar: 'from him who is (ho ōn) and who was (ho ēn) and who is to come (ho erchomenos).' The grammar is deliberately ungrammatical — Greek nouns would normally decline, but ho ōn is left in the nominative regardless of its grammatical position. This is a theological grammatical protest: the divine being cannot be made subject to human grammatical rules. God does not decline; He simply IS. John 1:1 uses a related construction: 'In the beginning was (ēn) the Word' — the imperfect tense expressing continuous, unbounded existence. The ōn of God is the foundation of all created onta (beings) — all other existence is contingent, derived, bounded; His is absolute, self-existent, eternal.