You'd use same form as the original return but check the box for amended return.  You'd need to set it up with the following -

1. Screen 59 / General Information and Federal (1040X):
1. Click on dropdown to create OR amended return;
2. Indicate year to amend;
3. Review and check applicable boxes under Miscellaneous Information;
4. Populate Explanation of Changes.
2. Screen 59 / Oregon Amended Return:
1. Populate entries for original taxes paid, refund, etc.
3. Update relevant entries for OR as if preparing an original return.