Oracle 18c

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CREATE FUNCTION get_keys(
  value IN CLOB
) RETURN SYS.ODCIVARCHAR2LIST PIPELINED
IS
  js   JSON_OBJECT_T := JSON_OBJECT_T( value );
  keys JSON_KEY_LIST;
BEGIN
  keys := js.get_keys();
  FOR i in 1 .. keys.COUNT LOOP
    PIPE ROW ( keys(i) );
  END LOOP;
END;
/
CREATE TABLE table_name ( value VARCHAR2(4000) CHECK (value IS JSON) );
INSERT INTO table_name (value) VALUES ('{a:100, b:200, c:300}');
1 rows affected

SELECT t.value,
       k.keys
FROM   table_name t
       CROSS JOIN LATERAL (
         SELECT '['
                || LISTAGG(COLUMN_VALUE, ',') WITHIN GROUP ( ORDER BY ROWNUM)
                || ']'
                  AS keys
         FROM   TABLE(get_keys(t.value))
       ) k;


VALUE
KEYS


{a:100, b:200, c:300}
[a,b,c]


SELECT t.value,
       '['|| LISTAGG(SUBSTR(path, 3), ',') WITHIN GROUP(ORDER BY ROWNUM) || ']' AS keys
FROM   table_name t
       CROSS JOIN LATERAL(
         SELECT JSON_DATAGUIDE(t.value) AS data
         FROM   DUAL
       ) d
       CROSS JOIN LATERAL(
         SELECT path
         FROM   JSON_TABLE(
                  d.data,
                  '$[*]'
                  COLUMNS(
                    path VARCHAR2(20) PATH '$."o:path"'
                  )
                )
       )
GROUP BY t.value;


VALUE
KEYS


{a:100, b:200, c:300}
[a,b,c]