A district attorney who prosecutes a case where there is less than a 50/50 probability that a crime has been committed, can be sued for malcious prosecution, assuming that the case is dismissed in favor of the defendant, and it can be shown that the DA's actions were taken for a "wrongful purpose."
It's the last element of the action that's the hard one to prove (wrongful purpose). However, if you really believe that the DA has gone out of his/her way to prosecute someone wrongfully, then you can complain to the Oregon State Bar Association (http://www.osbar.org/).
And, you can hire an attorney and sue the DA for malicious prosecution.
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