After successful installation, Is it possible create a database without DBCA?
Yes. First you need a parameter file (init.ora). The only mandatory parameter is db_name to have a fully working database. 

Steps

1 - Create the parameter file:

[myhostname@oracle $] vi $ORACLE_HOME/dbs/initorcl.ora

db_name='orcl'

:wq


2 - Export ORACLE_SID to reflect the db_name you which to create:

[myhostname@oracle $] export ORACLE_SID=orcl


3 - Login to sqlplus:

[myhostname@oracle $] sqlplus / as sysdba


4 - Issue the "startup nomount" command, pointing to the parameter file you have created.

SQL&gt; startup nomount pfile = '/u01/app/oracle/product/&lt;version&gt;/dbs/initorcl.ora'


5 - Issue the create database command

SQL&gt; create database;


6 - Run the post database creation scripts:

SQL&gt; @?/rdbms/admin/catalog.sql -- Create dictionary views

SQL&gt; @?/rdbms/admin/catproc.sql

SQL&gt; @?/sqlplus/admin/pupbld.sql


@ means "run an external script in sqlplus"
? is a wildcard character that represents the ORACLE_HOME path, so if you correctly installed the software and set the environment variables, it must work, otherwise provide the full ORACLE_HOME path.

You may also want to know the "whys" of each stage of the database creation process...

At step 1, you don't have a database (datafiles, controlfiles...), so in order to create a database, you first must have an instance, which is a set of memory structures and background processes, and for that purpose, the only need is a parameter file to startup the instance in nomount. To proceed to the mount stage, you need either a controlfile if the database already exists, or issue the create database command to let Oracle take care of creating the necessary files, tablespaces etc. After that, the database can be mounted and opened, but still the data dictionary views aren't populated. So you just need to run the post scripts to do the job.

Reference