OPM should be sending the EX their share of the distribution AND issuing their own 1099-R. If this is not happening then it is up to you to get OPM a copy of the order so that this can be done properly.
If you don't then YOU will get the only 1099-R for the total distribution and then YOU will pay taxes on the entire distribution UNLESS your divorce decree states the amount is considered alimony and the divorce was completed prior to 2018.
