The optimization process and recommendations:

1. Avoid Calling Functions With Indexed Columns (query line: 6): When a function is used directly on an indexed column, the database's optimizer won’t be able to use the index. For example, if the column `organization_code` is indexed, the index won’t be used as it’s wrapped with the function `NVL2`. If you can’t find an alternative condition that won’t use a function call, a possible solution is to store the required value in a new indexed column.
2. Avoid Selecting Unnecessary Columns (query line: 2): Avoid selecting all columns with the '*' wildcard, unless you intend to use them all. Selecting redundant columns may result in unnecessary performance degradation.
3. Avoid Subqueries (query line: 6): We advise against using subqueries as they are not optimized well by the optimizer. Therefore, it's recommended to join a newly created temporary table that holds the data, which also includes the relevant search index.
4. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.

Optimal indexes for this query:

ALTER TABLE `org_organization_definitions` ADD INDEX `org_definitions_idx_organization_code` (`organization_code`);

The optimized query:

SELECT
        * 
    FROM
        org_organization_definitions ORG 
    WHERE
        ORG.organization_code IN NVL2(:p_org_code, (SELECT
            REGEXP_SUBSTR(:p_org_code,
            '[^,]+',
            1,
            DUAL.LEVEL) 
        FROM
            DUAL CONNECT 
        BY
            REGEXP_SUBSTR(:p_org_code,
            '[^,]+',
            1,
            DUAL.LEVEL) IS NOT NULL), ORG.organization_code)