This is apparently late, but I just saw the questions. According to the instructions, you enter the amount on Sch A: instruction page 13
https://www.ok.gov/tax/documents/511Pkt-18.pdf
https://www.ok.gov/tax/documents/511-2D-2018-UF-VerCtl.pdf
Military Retirement
Each individual may exclude the greater of 75% of their retirement benefits or $10,000, but not to exceed the amount included in the Federal Adjusted Gross Income. The retirement benefits must be from any component of the Armed Forces of the United States.
Comment: The instructions are not very clear, but it appears you would enter into Sch A, 75% of the gross pension...but not to exceed the amount included the Federal Adjusted Gross Income. Thus it appears if your gross pension is $25,000, you could deduct $18,750...as long as $18,750 was included in your AGI of your Federal Income Tax return.
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