No, this is correct.
The reason is that that $300 cash contribution reduces your federal adjusted gross income. Line 7 on Oregon OR-40 is your federal adjusted gross income, which you now know already has the $300 removed.
So it would be double-dipping to also remove this as a charitable contribution.
This is why the Oregon instructions say:
"Note: If you're itemizing deductions for Oregon, you must omit this deduction from the charitable contribution you claim on Schedule OR-A. See the instructions for Schedule OR-A." See New Information at the OR-40 Instructions.
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