With Guava being a compile-scope dependency, we currently leak out a dependency to Guava 15 to any user of Oak. In particular, downstream projects might use Guava without having explicitly declared a dependency, and will get a narrow version scope (that is, for now 15).
If we change the scope to "provided", downstream users who do use Guava will have to declare their dependency explicitly.
The current recommended version range can be found in oak/oak-parent.xml as "guava.osgi.import".
When deploying Oak, make sure that a matching version of Guava is present in the class path (for instance, for now: https://mvnrepository.com/artifact/com.google.guava/guava/15.0)