If you're looking for anime similar to High School DxD OVA, you might like these titles. All recommendations are made by Anime-Planet users like you!
Basara Toujo is a high school student whose father has suddenly just remarried. His father then departs overseas leaving Basara with two new beautiful step-sisters. Little does he know, his new sisters, Mio and Maria are actually the new Demon Lord and a succubus!? Almost trapped into a life of servitude, Basara forms a reverse contract by accident and ends up becoming Mio’s master! Hijinks ensue as Basara finds himself in one ecchi situation after another. However, Mio’s life is in danger as she is pursued by demons and heroes!
Shinmai Maou no Testament is really similar than Highschool DXD. It's also an action-fantasy anime which have a similar setting, having 3 different factions which were previously at war (demons, angels and fallen angels in DxD and demons, heroes and gods in Testament) which have their eyes set on the main girl. Also said girl happens to be a red-haired demon. Needless to say it's quite ecchi (though I'd say Testament is way more explicit than DxD). If you enjoyed this one you'll like Shinmai Maou no Testament.