1.Successfully Complete ALL the MAIN Scoops (missions). Cases 1-8 + The Facts.
Rescue As many survivors as you can (doesn't need to be perfect and get them all) Prevent Mutiny or fulfill survivor requests if possible to keep them from leaving/dying.
There is no need to rescue any survivors for overtime mode to unlock, just the main missions. Thanks " Jon81D "
3.be AT the HeliPad you start from BY NOON of the last day (day4). after talking to isabella for the last time in the north plaza around 10AM head back to the Helipad and wait for the helicopter to arrive.
4.Cutscene + Achievement should pop
5.Credits roll, then Overtime Mode begins. Enjoy