depends on the complexity and volume of data.

sql*loader will work. But u need to spend your time in creating the flatfile, 'mapping' the columns to the oracle tables etc.

SINCE the data is already in an ORACLE DATABASE, we should make use of oracle functionalities within the database itself.

there is something folks call ETL (Extraction, transformation, loading).

use database links to talk from one database to another database.

then you can use simple insert statements or stored 
procedures to extract the data from source tables 
(source database) , transform the data (if there is 
difference in datatype, or fields being concatenated 
etc), and load into the target database (insert).

--
-- here i am connected to a database LAWT
--

dbadmin@constitution_lawt2 > select name from v$database;

NAME
---------
LAWT

--
-- lawp is a database link, that connects to another database LAWP
-- lets check it.

dbadmin@constitution_lawt2 > select name from v$database@lawp;

NAME
---------
LAWP

--
-- Lets do some querying in the remote database.
--

dbadmin@constitution_lawt2 > select table_name from dba_tables@lawp where owner='DBADMIN';

TABLE_NAME
------------------------------
DEPT
EMP
PLAN_TABLE
TABLE_STATS

--
-- lets create a table (locally in lawt) from the remote db(lawp).
--

dbadmin@constitution_lawt2 > create table another_emp as select ename,sal from emp@lawp;

Table created.

dbadmin@constitution_lawt2 > desc another_emp;
 Name                                                  Null?    Type
 ----------------------------------------------------- -------- ------------------------------------
 ENAME                                                          VARCHAR2(10)
 SAL                                                            NUMBER(7,2)