On your 2018 federal tax return you can deduct the Oregon state income tax that you paid in 2018, or that was withheld from your pay in 2018. It doesn't matter what year the tax was for. What matters is when you paid it or when it was withheld.
The total deduction for state and local taxes is limited to a maximum of $10,000 per year ($5,000 if you are married filing separately). The maximum applies to the total of real estate tax, personal property tax, and either state and local income tax or state and local sales tax.
State income tax is an itemized deduction. It will not make any difference in your federal tax unless your total itemized deductions are more than your standard deduction.