I don't know how to have my federal annuity income for tax purposes be what is shown on the 1099-R in box 2a. Does it have something to do with the RMD discussion?

When you are entering the form, are you filling in box 2a or are you letting the program figure it out? Go ahead and fill in the correct amount if you know it. When you look at your 1040, you should see the taxable amount on line 5b. Be sure to mark that it was all RMD as you go through the questions after input.

Also, make sure you select the right form.

After you start into the 1099-R entry area, I'm pretty sure your form (from OPM) is either a CSA-1099-R (OR CSF-1099-R for a surviving spouse).  Make sure you've chosen the correct sub-form type...i.e. I suspect, not a regular 1099-R.

Now a TSP distribution? That would be a regular 1099-R.