I created UNIX groups - oinstall, dba and UNIX user - oracle for the installation of Oracle 10g. But I might did something incorrectly. Oracle user account didn't created properly. How to remove these UNIX groups and user so that I can start over again to create them properly. Thanks.

You can use the userdel command, or just fix what's wrong by editing /etc/passwd, /etc/shadow, and /etc/group. What's the problem you are having?

System Shock:

I created UNIX user (oracle) and groups (oinstall and dba). Then I created oracle base directory as /u01/app/oracle and /u01/oradata. Then I chown them to oracle:dba and chmod them as 755. Then I want to login as user oracle. I did su - oracle, system told me that No Directory! What is the problem here? Did I miss some steps or something wrong? I tried to delete user and group completely, then re-create them to make it work. Please advise. Thanks.

yes, you forgot to create/set home directory for the oracle user.

You can edit the appropriate field in /etc/passwd to correct this.

I wanted to login as oracle user to set up oracle home directory. Now I even can not login as oracle user. Can I set home directory for oracle user as root user? How to do it? could you tell me more? Thanks.

Check in /etc/passwd if oracle has a defined home directory. If not, you can either use usermod to give it a home directory, or edit /etc/passwd. If, say, you made /u01/app/oracle the oracle home directory, then you should see an entry in /etc/passwd like:
If you use the usermod command, the following should do it

System Shock:

Thanks so much. You are guru. I followed your instruction to make oracle home diretory right. Now I can login as oracle user to do next step. Thanks and Happy Holiday to you and others in this forum.