Do I need to show "fault," such as proof of adultery, in order to get a divorce in Oregon?

No. Oregon is a no-fault divorce state, which means you are not required to show whose fault it is or even why the marriage is ending. In Oregon, all you need to tell the court about why your marriage is ending is that you and your spouse have irreconcilable differences. In other words, if your marital problems cannot be resolved and you want to be divorced, a judge will grant you a divorce.