I passed the cars on the right. This is legal by Oregon law ORS 811.415 and is safer than passing a line of cars on a 2 lane highway where oncoming traffic typically moves around 65mph -- especially since I do need to worry about being sideswiped or mowed down by someone else wanting to pass from behind. The cars would have been crazy to expect me to do anything different since this amounts to me holding my line and them holding theirs.

However, combines are very wide so they took the entire shoulder to keep them out of the oncoming lane. That made it impossible to pass them on the right, so I overtook them on the left.

In some cases, I had to wait for oncoming traffic before passing. However, it only takes me a couple seconds to clear a combine if I'm hammering, and I don't need nearly as much space as a car. As a result, there were many passing opportunities that cars could not use that I could.