Is it possible to increase Oracle execution plan output indentation?
You can use PLAN_TABLE for such result as following:

Your statement for generating the execution plan

EXPLAIN PLAN 
SET STATEMENT_ID = 'mystatement1'  -- give your statement a meaningful ID so that we can fetch it from PLAN_TABLE using it
    FOR
SELECT
    *
FROM
    ACTIVE_USERS;


Now, Fetching the execution plan from PLAN_TABLE

SELECT
    ID,
   -- LPAD(' ', LEVEL - 1) -- default format
    LPAD(' ', LEVEL * 2) -- Indentation is done using this
    || OPERATION
    || ' '
    || OPTIONS
    || ' '
    || OBJECT_NAME "Operation",
    OBJECT_NAME   AS "Name"
FROM
    PLAN_TABLE
CONNECT BY PRIOR ID = PARENT_ID
           AND PRIOR STATEMENT_ID = STATEMENT_ID START WITH ID = 0
                AND STATEMENT_ID = 'mystatement2'
ORDER BY
    ID;


Output:



You can add more columns into result according to your need and also add more spaces using LPAD in the above query.

Cheers!!