Is it against the law in Oregon to use a buck knife in self defense?

Me & my boyfriend went to the bar across the street tonight. We were trying to just relax and these 2 woman walked up to use. They immediately began to try to act drunk and too friendly if you get my drift. I knew what they were up to from the beginning. I just wanted to relax and eat my dinner and when they asked to sit with us I politely said "I would prefer if you didn't." They sat down anyways and began to chatter about this and that and act innocent. They mentioned not having enough money to get beer, even though one of them had a beer. They even tried to claim they were charged $4.00 for a $1.95 beer. I went out for a smoke and they followed me. One of them told me they were coke heads. Her freind then began to insult my boyfriend calling him a fat lil potato and saying "I bet hes stupid he looks young". When I went back in I told my boyfriend "lets go" These girls would not give us an inch. We went outside and one of the other guys who I spoke with distracted them. We left and headed toward home. We cut through an ally and I heard then running behind us. We managed to get to our apartment without an incident. I have no doubt if my boyfriend had been by his self they would have mugged him. Anyways, I was wondering if we encountered these 2 again and I was forced to hit one of them with the handle of my buck knife, would it be self defense?