The nexus-oss-webapp bundle was renamed to nexus-bundle-template in Nexus 2.6. This change was made to make it clearer that it is not an official Sonatype distribution, but a subset of the free version of Sonatype Nexus.
The nexus-bundle-template is otherwise the same as nexus-oss-webapp used to be.
The official Sonatype Nexus OSS distribution is built from a private repository. It's Maven coordinates are com.sonatype.nexus:nexus:<version>:bundle:zip and is available from https://repository.sonatype.org for Sonatype customer logins only or can be downloaded by anyone from the Sonatype website.