In case you have an older Java 6 application and want to switch to Java 7 code with goodies like diamond operator and multi-type exceptions:

touch .openshift/markers/java7
git add .openshift/markers/java7

Also make sure you don't have it ignored in .gitignore like I had.

Then in pom.xml

<plugin>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.0</version>
    <configuration>
        <source>1.7</source>
        <target>1.7</target>
    </configuration>
</plugin>

Then git push, and you're done.