In Oklahoma, can we not take the itemized deductions if they aren't used on federal. What is the 511-d for then?

Yes, you are correct. If you claimed the standard deduction on your federal return, you must claim the Oklahoma standard deduction.  If you claimed itemized deductions on your federal return, you must claim Oklahoma itemized deductions

https://www.ok.gov/tax/documents/511Pkt-18.pdf pg 7

Schedule 511-D is for reporting itemized deductions because if you itemized on your Federal return you must claim Oklahoma itemized deductions.