Yes, since you want it to post to your 2017 state return, report it as paid in 2017. Be sure to let the account administrator know that you want it to be posted to 2017. They may or may not adjust their records. You can claim it in 2017, but not again in 2018. Keep records with your tax file. 

Oregon

• State tax benefit: Contributions and rollover contributions up to $2,330 (for 2017) for a single return and up to $4,660 for a joint return are deductible from Oregon state income tax.
• Contributions must be made to an Oregon 529 plan in order to qualify
• Deadline: April 17, 2018

https://www.savingforcollege.com/articles/6-states-where-you-can-still-claim-a-prior-year-529-plan-t...

The referenced article no longer lists Oregon as a state that allows contributions for the previous year up until the tax filing deadline. I'm not sure whether this is still correct, although the Oregon College Savings Plan itself has one page suggesting it is: https://www.oregoncollegesavings.com/tax-benefits