MongoDB Maxing CPU Usage
Strange indeed. 

That local.oplog.rs is journaling what is enable (default) at 64 bit systems (32bit default value is false). So, running time for that thread should be same than your mongod uptime. 

But journaling should not rise CPU usage. When you have high CPU usage, what mongotop and mongostat shows? What is happening in the system?  

If you look result of top-command, "who" is eating most CPU power (one or more mongod processes?) and what is highest %-value us(er), sy(stem), ni(ce), id(le), wa(it)?