vi /etc/selinux/config      #disable selinux
yum -y update
yum -y install wget httpd mysql-server mod_perl perl-core procmail perl-Archive-Zip perl-XML-LibXML perl-XML-LibXSLT perl-XML-Parser perl-Crypt-SSLeay perl-LDAP perl-Net-DNS perl-TimeDate perl-Template-Toolkit
wget http://ftp.otrs.org/pub/otrs/RPMS/rhel/6/otrs-5.0.20-01.noarch.rpm
rpm -ivh otrs-5.0.20-01.noarch.rpm
service httpd start
vi /etc/my.cnf
## add this line under mysqld section
max_allowed_packet=20MB
innodb_log_file_size=512MB
## end
service mysqld start
mysql_secure_installation
iptables -A INPUT -p tcp -m state --state NEW -m tcp --dport 80 -j ACCEPT     #allow tcp 80, HTTP
chkconfig --levels 2345 httpd on
chkconfig --levels 2345 mysqld on

Access http://yourserver/otrs/installation.pl from your browser to start installation process.
After installation process, run this command in shell to run OTRS Daemon.

su -c "/opt/otrs/bin/otrs.Daemon.pl start" -s /bin/bash otrs