Here is short installation HOWTO for Oracle Database 12c Release 2 for Redhat 7, Oracle Linux 7 (and possibly CentOS 7 which is not officially certified and supported distribution). This post covers Oracle Database software installation (only). In case you want to use ASM or other Grid features and you don't have Grid Infrastructure 12c R2 installed refer to my previous post: http://ivan.kartik.sk/index.php?controller=post&action=view&id_post=76 If you don't need ASM or GI features you can continue with this part directly.
This post also contains some steps which were used during Oracle Grid Infrastructure 12c R2 installation, so you can skip those if you followed my GI installation HOWTO.

1. OS preparation

# - stands for "root" action
$ - stands for "oracle" (owner of Oracle software)

Note: You can skip this step if you have installed the Grid Infrastructure Software.

# groupadd dba
# groupadd oinstall
# useradd -g oinstall -G dba oracle

Create ORACLE_HOME directory. (Note: In my example I use /opt/oracle/12201 directory, change it to your preferred location):

# mkdir -p /opt/oracle/12201
# chown -R oracle:oinstall /opt/oracle

2. Install required packages

Note: You can skip this step completely once you have installed the Grid Infrastructure Software (as described in my previous post related to Grid Infrastructure installation).

First install required packages (use this command if your computer has access to Internet or you have registered and enabled your media as yum repository).

Add/set kernel parameters to/in /etc/sysctl.d/99-sysctl.conf file:

fs.file-max = 6815744
kernel.sem = 250 32000 100 128
kernel.shmmni = 4096
kernel.shmall = 1073741824
kernel.shmmax = 4398046511104
kernel.panic_on_oops = 1
net.core.rmem_default = 262144
net.core.rmem_max = 4194304
net.core.wmem_default = 262144
net.core.wmem_max = 1048576
net.ipv4.conf.all.rp_filter = 2
net.ipv4.conf.default.rp_filter = 2
fs.aio-max-nr = 1048576
net.ipv4.ip_local_port_range = 9000 65500

Execute sysctl -p command to apply settings:

# sysctl -p

Add/set limits to/in /etc/security/limits.d/99-oracle-rdbms.conf file:

oracle   soft   nofile    1024
oracle   hard   nofile    65536
oracle   soft   nproc    16384
oracle   hard   nproc    16384
oracle   soft   stack    10240
oracle   hard   stack    32768
oracle   hard   memlock    134217728
oracle   soft   memlock    134217728

4. Software download and install

Download installation archive (linuxx64_12201_database.zip) from http://www.oracle.com/technetwork/database/enterprise-edition/downloads/oracle12c-linux-12201-3608234.html

Unpack the installation archive and start the installer:

$ unzip linuxx64_12201_database.zip
$ cd database
$ ./runInstaller

Next steps after installation

In case you want startup your Oracle Database(s) automatically and you don't have Oracle Grid Infrastructure installed, you can follow the steps described in the post Configuring automatic startup of Oracle Database under systemd on RHEL 7/OEL 7/CentOS 7 at http://ivan.kartik.sk/index.php?controller=post&action=view&id_post=65