OpsCenter 1.4.1 is not tested against cassandra 0.7.x, however it should work correctly. One likely problem is that the jmx port in cassandra 0.8 and higher changed to 7199. OpsCenter 1.4.1 defaults to using that port. On a 0.7.x cluster the port will be 8080. You can change this setting in your opscenterd.conf file. Once you change the setting and restart the main opscenter process, the agents should get the new port and start connecting to cassandra.
If that is not your problem, I would advise checking the agent log on your machines to see if there are any errors. It can be found in <install_dir>/log/agent.log or /var/log/opscenter-agent/agent.log