The problem is as much oversight as there is they intentionally dissemble and use twisted definitions when testifying before the congressional oversight committies. In 2013 the claimed their PRISM and associated wide scale surveillance activities didn't 'collect' information about or surveil american citizens because respectively they didn't task or view the information and didn't listen in on the actual content of the conversations. It's impossible to properly oversee an organization that is that circumspect with the people overseeing them.