I dont understand what do you mean exactly. If what you are saying is

in (1, 2, 3)

is same as

1 or 2 or 3

then yes it is correct but in a query plan.

Or you mean in the query plan you see a query is decomposed into several and concatenated at the end?

What I said an in-list is not same as OR in CBO terms. An in-list iterator is not resolved as a in (***) as that using RBO

I think we have some missunderstanding, when I said CBO uses inlist or OR depends I mean they are not resolved the same.

regarding concatenation

Code:
explain plan for select * from emp where deptno in (10, 20);

SQL> explain plan for select * from emp where deptno in (10, 20);

Explained.

SQL> @plan

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------

----------------------------------------------------------------------------
| Id  | Operation                    |  Name       | Rows  | Bytes | Cost  |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |             |       |       |       |
|   1 |  CONCATENATION               |             |       |       |       |
|   2 |   TABLE ACCESS BY INDEX ROWID| EMP         |       |       |       |
|*  3 |    INDEX UNIQUE SCAN         | EMP_IDX1    |       |       |       |
|   4 |   TABLE ACCESS BY INDEX ROWID| EMP         |       |       |       |
|*  5 |    INDEX UNIQUE SCAN         | EMP_IDX1    |       |       |       |
----------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - access("EMP"."DEPTNO"=20)
   5 - access("EMP"."DEPTNO"=10)

Note: rule based optimization
Using CBO it always tries to use inlist, at least from 9i upwards