The ORS 165
.543 charge sounds like B.S. to me.
165.543 Interception of communications.
(1) Except as provided in ORS 133.724 or as provided in ORS 165.540 (2)(a), any person who willfully intercepts, attempts to intercept or procures any other person to intercept or attempt to intercept any wire or oral communication where such person is not a party to the communication
and where none of the parties to the communication has given prior consent to the interception, is guilty of a Class A misdemeanor.
(2) As used in this section, the terms “intercept” and “wire or oral communication” have the meanings provided under ORS 133.721. [1983 c.824 §3]
Italics mine. This statute pertains to listening in other people's conversations, not recording one in which you're taking part.
I've found the statute that might stick, but note that it contains a bunch of exceptions, one or more of which might be applicable in your situation. But IANAL.
Regarding assistance, have you considered the ACLU? I know, they're not brilliant where gun rights are concerned, but since the cops are resorting to charging you for recording your public conversation with them, this has wider ramifications. I think the ACLU will disagree with the notion that the cops have the right to bust you because they don't like you recording their interaction with you.
You can request assistance from the ACLU of Oregon here