I tried to install Nexus 1.9.2.2 OSS on my Tomcat 7.0.20 with the new Oracle JDK 7. This was not a big success. Ok first of all make sure to use the unpacked version of the nexus war or else it wont be able to find the NEXUS_PLEXUS_WORK environment variable.

Second, you need to patch the unpacked war. Strip out the xstream-1.3 and add the latest snapshot. Also add the latest xmlpull.org API jar to the WEB-INF/lib of the unpacked nexus war.

More info see this JIRA ticket.