The rules on recording and filming in Oregon are summarized athttp://www.rcfp.org/reporters-recording-guide/state-state-guide/oregon
Oregon is a "one party consent" State as to electronic communications, but requires the consent of all parties to tape an in person conversation.
Police vehicles commonly have dashboard mounted cameras, but it is not usual for individual officers to wear hidden cameras or sound recording devices.
You wrote: "I would like to know if in this country of ours one can be videoed by the police anytime the police feel like it without letting us know." The answer varies among the States but in general a police officer (or anyone else) is free to shoot videos on public streets. Many private businesses have outdoor security cameras, so we are all being videotaped every time we go to a city. I don't like it, but it is a fact of modern life.
This cuts both ways. The officers who assaulted Rodney King
would never had been tried if not for the videotape shot by George Holliday. So while the police can point a video camera at people on public streets, those same people can also point video cameras at the police on public streets or other public locations, such as train stations, as in the Oscar Grant
Some people wear large sunglasses and floppy hats when they go to a city if they really do not like being videotaped.
I hope this information is helpful.