What are my rights concerning response to assault? Can I use physical force in my defense?

I am not familiar with Oregon law, but generally speaking a claim of self-defense only allows you to use that degree of force reasonably necessary to defend yourself from death or serious bodily injury.  If the assault has ended, and you are no longer in danger of further injury, you can't launch a revenge attack.  Also, the degree of force used in self-defense has to be reasonable.  For example, if someone shoves you with their hands, responding with a gun or knife would be excessive force and would not support a self-defense claim.