In general, yes, you can apply all of your RMD as tax withholding. However, some custodians place an upper limit on the percentage of your distribution that they will allow to be withheld, so check with the custodian.
Tax withholding is preferable to paying estimated taxes because tax withholding, by default, is treated as applying in equal parts to each of the tax quarters of the year, while estimated tax payments are credited when paid. Applying a large part of the distribution as tax withholding can help reduce or eliminate an underpayment penalty that you might otherwise have for underpayment in earlier quarters of the year. This can also help eliminate the need to annualize your income on Form 2210.
