Can the state of OR suspend a WA state driver's license for the minor traffic violation of not wearing a seat belt if the infraction happened in OR?

Yes, if you failed to respond to the ticket.

I would check with the Washington DMV to make sure, but I believe that the Oregon DMV would have to advise the Washington agency that they had been advised by the local court that the Washington driver had been found guilty of an infraction that would mandate license suspension in Oregon. At that point, you need to check with the Washington DMV to find out (a) will Washington automatically honor Oregon's request? and (2) what does the driver have to do to get the license reinstated (fines, classes, etc.).