Do you get the entire payment and then you send money to your ex? Because if this was a divorce issue then OPM should have been directed to pay her portion directly to her.  And when was the divorce?

Opm sends her the check.  There is a notation on the 1099r that she was paid $'s under court ordered apportionment and that the taxable amount is unknown 

If she gets paid directly then she should get a 1099-R in her name for her portion of the payment ... if this is not happening then get that corrected ASAP.  Only your portion of the retirement payment should be showing on the 1099-R form and reducing the taxable portion on the tax return can get you an IRS letter to deal with later. 

If the divorce happened before 2019 then you will report ALL of the box 1 amount in box 2a as taxable then deduct her portion as alimony paid.  This will net you out the desired figure for 2020. 

If the divorce happened after 2018 then you must report all the income and there is no deduction for the alimony paid.  So get this fixed immediately ... OPM is good about QDRO orders and doing the paperwork correctly.