If you're as confused about what OD is as everyone else then director Hideo Kojima has a message for you. While he won't say what it is outright, he tells gamers to watch the teaser trailer "100 times" and they'll maybe see a "big hint."
In an interview with Nikkei Xtrend, translated by GamesRadar+, Kojima says that OD is a kind of media "like no other," despite being made entirely in a game engine. "I think when you first see OD it looks like horror," he says. "But the system it uses isn't ordinary. Everything is made in a game engine, and a game is a game, but it's still a system like no other. That's why we built a completely new medium together with Microsoft."
