From the Bike Law Blog: Are bicyclists required to give a signal when passing pedestrians?
Yes, a bicyclist, when passing pedestrians on a sidewalk, must give “an audible signal”. ORS 814.410(1)(b).
Where it gets tricky is in the definitions.
What is a sidewalk?: Sidewalk has a pretty hefty definition in the Oregon Vehicle Code but the biggest clue you are on a sidewalk is whether or not it it runs alongside a road, street, or highway, and is capable of being used by a pedestrian. This means multi-use paths like Portland’s Springwater Corridor or the paths in parks are usually not sidewalks. In that situation an audible signal is only required by custom and respect, not the law.
What constitutes an audible signal?: There is no definition or standard for an audible signal in Oregon law. Many people use bells or a vocal announcement that they are passing, and both would certainly qualify as audible signals.