TSP Tax / Cost Basis?

OPM wouldn’t be able to provide you with any information about your TSP RMDs, as they do not administer the Thrift Savings Plan.
I’m assuming that by using the name “retiree2010”, that you retired in the year 2010. If that is the case, your TSP should consist of only pre-tax money and all of the RMD will be taxable at your rate for ordinary income.
Your federal annuity payment does consist of both post tax money (your contributions) and pre tax money (the government’s contribution and earnings on all contributions). If OPM does not provide a breakdown of the taxable versus tax free portion of your annuity, you should be able to compute it yourself using information found in IRS Publication 721.

Thank you for your response. That confirms what I had found in my research.

I do have an additional comment. If we already paid taxes on our salary before investing in the TSP, wouldn’t that portion of the RMD distribution be tax free, and we would only own on the interest earned?

If you had already paid taxes on your TSP contribution, then the part of your distribution that was from post-tax money would be tax free. Beginning in 2012, the TSP offered a Roth option that allowed you to contribute with post-tax dollars. Any qualified withdrawals would have the earnings tax free.

If you were contributing (both regular and catch-up) to the traditional Thrift Savings Plan you would have no cost basis in the TSP, as all of your contributions would have come from pre-tax dollars and the taxes on your earnings would have been deferred. You would have to pay federal income tax (at your rate for ordinary income) on everything you withdrew.
If, on the other hand, you contributed to the Roth TSP, your cost basis would be the amount of your Roth contributions and, as long as your withdrawals were qualified, you would not pay any tax on your Roth withdrawals.
The TSP currently requires that all withdrawals be done proportionally between your traditional and Roth balances.