They’re different than in Pathfinder, that’s for sure. “The Equipment chapter in the Player’s Handbook notes that a weapon with the Reach property increases your reach by 5 feet for the sake of attacks with it, including opportunity attacks. This means that, if you’re wielding a halberd, for example, a hostile creature only provokes an opportunity attack from you when they move from a space within 10 feet of you to a space further than 10 feet from you. Counterintuitively, this means wielding a weapon with Reach may actually limit your options for opportunity attacks, in some cases!”
John V for the win on this one – thanks!