According to the IRM (https://www.irs.gov/irm/part5/irm_05-008-002)
126.96.36.199.1 (05-25-2018) Determining Processability (of Offers In Compromise)
Under #2 Unfiled Tax Returns: There’s a note that says “Consider the tax period as compliant if a valid extension is on file.”
Other research suggests that the IRS will, nevertheless, contact the taxpayer requesting the return or proof that nothing will be owed. This essentially means prepare the return.