When triggering more than 2 concurrent jobs with the Operations orchestation plugin, only 2 run at the same time.
i went over the code in github and i think this class is causing it:

https://github.com/jenkinsci/hp-operations-orchestration-plugin/blob/master/src/main/java/com/hp/mercury/ci/jenkins/plugins/OOBuildStep.java

line 40 : import org.apache.http.impl.conn.tsccm.ThreadSafeClientConnManager;
line 111 : httpClient = new DefaultHttpClient(new ThreadSafeClientConnManager());

the "ThreadSafeClientConnManager" is limited by default to creating only 2 connections to a host at the same time.

you might want to try something like :
line 111 : httpClient = new DefaultHttpClient(new ThreadSafeClientConnManager().setDefaultMaxPerRoute(10));