Ok. What I meant was:
1. Install RHEL O/S on the new server and make sure the IP of the new server is same as old as for ex: if old server IP is 188.8.131.52 then make sure to use 184.108.40.206 IP on new server. Since IP of both machine is same, you can not add new server in the domain else you will get IP conflict issue.
2. Install Oracle software on RHEL machine and create user with default tablespace on RHEL as per AIX. For ex: on AIX if you have a user called my_app with application_user as its default tablespace then create my_app user on RHEL machine with default tablespace as application_user.
3. Now start exporting users from AIX server and ship the dump file to RHEL server where import them using impdp utility
4. Once these steps are done, remove AIX from network and add RHEL machine into the network.
You can also check the MOS document suggested by Mustafa.
Thanks for the reply, but I didn't understand anything from your answer, my problem is I have Oracle database which is running on AIX machine. I want to migrate the same database to a Linux Machine.
Your problem is the endeaness: AIX is using big-endian format vs Linux is using little-endian format. therefore the datafiles are incompatible and needed to be converted. This can be acchieved using RMAN with CROSS PLATFORM TRANSPORTABLE TABLESPACES or as already said using Data Pump Export / Import. It isn't possible to simply copy the datafiles over to the new location.
To minimize downtime you might want to use a replication solution like GoldenGate or SharePlex (from DELL) which captures all changes during the migration process and posts it to the new location as soon as your main database is available.