Total Recommendations: 4
They both have the same genre: "Slice of bread". Oh, and it's a moe anime with moe characters doing moe stuff.
First of all, both are OVAs. In Nozoki Ana, it's the girl who plays around with the MC; in Nana to Kaoru,it's the girl. Both Kaoru and Emiru change their personalities when in school, and they're both secretly perverts. Then the MC soon gain feelings from the other. There's romance there somewhere... It's not hentai, but close enough.
There're a lot of similarities, like the 3D graphics, the MC's search for identity, and the emotionless personality of the characters. Also, other scenes take place in other worlds as unearthly beings fight each other while humans communicate to them. And, of course, Weapon-action fight scenes!
Other than being a harem, both anime takes place in a normal city. Normal guy gets dragged by an energetic heroine and someone's out to assassinate him. The protagonist is also expected to be a great man someday. Oh, and the heroine is of royalty. There are plenty of characters with similar personalities, like a tsundere, a sister who does all the cooking, cleaning, etc. Finally, it deals with supernatural phenomenon (one is sci-fi, the other is magic).