One of the Oracle Enterprise Manager 12c OMS installation prerequisites is to increase the number of file descriptors to at least 4096.
As root, edit the /etc/security/limits.conf file and add the following two entries:
<UID> soft nofile 4096
<UID> hard nofile 4096
As the installing user, i.e. oracle, confirm the new file limits by typing:
# su - oracle
$ ulimit -n
$ /bin/sh -c "ulimit -n"
Note: Substitute oracle with your installing user, i.e. su - my-installing-user to su to the installing user.
The value should be greater than 4096.