YugabyteDB 2.8

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create table values (_jsonb jsonb);
CREATE INDEX values_id_idx ON values (((_jsonb -> '_id'::text) ->> '$o'::text));
explain
select * from values where (_jsonb->'_id')::jsonb->>'$o' = '507f1f77bcf86cd799439011';

QUERY PLAN

Index Scan using values_id_idx on "values"  (cost=0.00..5.27 rows=10 width=32)

  Index Cond: (((_jsonb -> '_id'::text) ->> '$o'::text) = '507f1f77bcf86cd799439011'::text)