What oversight would you be referring to here? Does someone (not chosen by the people in charge of the NSA, up to and including the President) come in to periodically audit the NSA and ensure that they are only spying on Americans when they have a warrant? Do their computers even keep logs to enable such auditing?

The problem here is that you have a secret agency that is saying, "Look, really, we have these policies and we promise that we follow them. Of course, you cannot come in and check to see if we are following them, because our operations are a secret." It does not help that whistleblowers are considered to be traitors in this situation. How exactly do you think the rule of law works?