I'm trying to run an oracle database in virtual box under oracle linux
Had the same issue. Turned out to be a firewall issue. Here is how I turned it off:

As root in terminal:
chkconfig iptables off
chkconfig ip6tables off
service iptables stop
service ip6tables stop

Then modify /etc/selinux/config and set SELINUX=enforcing to SELINUX=disabled.

A more elegant solution is to open the 1521 port in the firewall. You can do that in the System -> Administration -> Firewall menu. Add 1521 (tcp) under "Other ports".