I live in Oklahoma and receive a civil service retirement pension (from USPS), is it considered a civil service retirement benefit in lieu of social security
Depends on how you receive them.
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 AGI.
Each individual may exclude their retirement benefits, up to $10,000, but not to exceed the amount included in the Federal Adjusted Gross Income. (To be eligible you must have retirement income in your name.) The retirement benefits must be received from the following: the civil service of the U.S..