Ulimit

/etc/security/limits.conf
* soft nofile 65535
* hard nofile 65535
/etc/pam.d/common-session*
session required pam_limits.so

verify
# su – oracle
$ ulimit -Hn
$ ulimit -Sn

# vi /etc/sysctl.conf

Append a config directive as follows:
fs.file-max = 500000

# sysctl -p