From Wikipedia:
“Under current overtime rules adopted in 2012, “teams . . . have the opportunity to possess the ball at least once in the extra period unless the team that receives the [first] overtime kickoff scores a touchdown on its first possession”. The game can also automatically end on a safety. If the team that received the first opening kickoff instead scores a field goal, the other team has an opportunity to tie or surpass that score; if they are able to tie the score, the next team to score any points wins.”