Not sure, but I think this is a 8 minutes long wannabe trailer pretending to be an OVA. It's like the creators were like "Hey lets try to turn this manga into an anime and see if people like it... and then get them all hyped and make the full anime!" It's literally just parts from the Anime. Can't say i enjoyed it watching it after i finnished the anime, it would have been better to watch it before, as a trailer. So if you're thinking of watching Mirai Nikki, watch this "teaser OVA" first to get teased or something. So watch this astrailer. But the anime is good so 10/10. (Jk, i actually rated it 7, it's a nice teaser.)