How to Change Unix user without su

What are the restrictions?
There are different ways, must you run as the user oracle presumably to run sqlplus / sysdba?
See if sudo is a viable option.
su requires the password of oracle account, while sudo uses your password to elevate rights if configured /etc/sudoers

Benefit of sudo is that data from the sudo session is logged.

Assuming you are allowed access to sudo, it can be set to whitelist binaries, and therefore limit access.  Of course, that depends on which binaries are allowed, since you can get full access if the wrong binaries are included.

For password-related tasks like this, "expect" is a useful tool.  You can get it to log on to the remote machine, run the "su - oracle" command, send the password, then use the "interact" expect comand to become interactive.  That also allows you to have expect watch the data stream and execute its own form of macro (I used to have a system which required 5 logins to get through the network to the machine I wanted on a customer site - logging out of those 5 machines again took ages, so I had "expect" looking for a particular sequence in my typed text and it would then log out of the machines cleanly to get back to my original machine).

su still requires the knowledge of a shared account password where sudo does not.
In the case of a shared password sudo without asking for a password might be preferable.

can be done with:
%useoracle ALL = ( oracle ) NOPASSWD: /bin/bash

not exactly my cup of tea... YMMV.
Another option for remote systems is using ssh  and using passwordless login using certificates.
Which requires the possession of a certificate (which might be password protected if needed)

Which OS? If AIX you can use rbac ( role-based access control). If so, tell me if you need help on this.

Thanks to everyone who participated and answered this question.
my OS is not AIX but solaris and also i have all the passwords including root and oracle but my role is oracle dba.
Earlier we used to login directly as oracle but recent audit advised to use own individual accounts and then su to oracle.
i was thinking i will put oracle password in .profile or some other way to avoid this second step

So, to recap, you're moving to individual accounts with su/sudo because a security audit recommended it, and you are responding by removing the security benefits this change provides.

Keep us updated on your next audit.

i would like to see the comment from the auditor on remark #a40822224 as well.
As the whole exercise most probably is meant to enhance security and not lower it.