The final rule was release today, March 9, 2016, this document is unpublished on the Federal Register, but on 03/11/2016 it is scheduled to be published and available. Until then, you can see the pre-publication PDF version here: Final OPT STEM rule. The rule will go into effect in May. The highlight of the new rule will be that certain foreign students in F-1 status with degrees in science, technology engineering or math (STEM) will be able to extend their initial 12-month OPT period by an extra 24 months, for a total of 36 months.
DHS has stated that 17-month STEM OPT work permit issued before May 10 will remain valid until it expires, but that beginning on May 10, certain students will have a chance to apply for an additional seven months of OPT.