For those using Ubuntu 14.04 LTS that does not have a OpenJDK 8 package in the official repository.
Webup8 Oracle Java8 Installer
sudo add-apt-repository ppa:webupd8team/java -y
sudo apt-get update
sudo apt-get install oracle-java8-installer
To automatically set up the Java 8 environment variables
sudo apt-get install oracle-java8-set-default
Check it
java -version
This also can be used update OpenJDK 8 via sudo apt-get install oracle-java8-installer it will clean up the old install. For those running Ubuntu 14.10 and up use sudo apt-get install openjdk-8-jre.
There are performance improvements to be had running a server with Java 8 not to mention developing mods. The only reason from an admin perspective to justify using an older version of Java is if a mod requires a lower version of Java. That is rare nowadays to the point Java 8 should be default in my personal opinion for MineOS.