The Treasury department has sent its proposed regulations regarding opportunity zones to the Office of Management and Budget for review.
As part of the Tax Cuts & Jobs Act, the proposed opportunity zones rules are subject to an expedited 10-day review period (usually 45 days) by the OMB. Once approved, Treasury will release its guidelines for a public comment period before issuing the final rules.
Expect the Treasury guidelines to be available for public comment in late September or early October.
Update 10/19/18: The IRS has published the first tranche of proposed Opportunity Zones regulations.