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After there + to be the relative pronoun "who" may be omitted:-eg:-There is oz (who) is terrible.
In relative pronoun in defining clause:-eg:-This is oz (who) is terrible;
is the relative pronoun used as the subject of oz the person eg:-Are you seeing oz (who) is terrible?=terrific, adjective, causing fear or terrible=eg:-Are you seeing the terrific (adj) oz ?