If you are referring to A3 on OK 511-A, please read the instructions:
"Federal Civil Service Retirement in Lieu of Social Security
Each individual may exclude 100% of their retirement benefits
received from the Federal Civil Service Retirement System
(CSRS), including survivor benefits, paid in lieu of Social Security
to the extent such benefits are included in the Federal Adjusted
Gross Income." Please see page 13 at www.ok.gov/tax/documents/511Pkt-18.pdf
This benefit refers specifically to federal government employees who paid in the CSRS instead of paying into the Social Security System.
Sorry, this does not apply to you, but please look at the instructions for A6 on the same page - I believe that line may refer to you.