I want to know if am been hired as an independent Oracle database administrator in a small firm, do I have to use Oracle universal installer only to create and manage their database or is there any special software recommended by Oracle corporation for that?
I use the OUI (Oracle Universal Installer) to install client software as well as the DBMS (database management software). There is really no other method in which to install oracle's software. Once it's installed, it is possible to copy the files in Oracle Home elsewhere... but it had to have been installed first in your 'master copy'.
However, if you want to create the database, then you don't have to use the OUI. This can be done with CREATE DATABASE scripts as well as manual methods of creating the listeners, tnsnames, etc. Managing the database can be done with scripts and other software such as OEM.
The motto of an independent oracle consultant could very well be: "Have scripts, will travel".