Is Maken-Ki! OVA hentai?
No
Rated R+ (Mild Nudity) on MyAnimeList and not tagged as Hentai.

OVA · 2012 · 2 episodes · R+ - Mild Nudity
Action, Ecchi, Harem, Martial Arts, School, Super Power
OVA episodes bundled with the 8th and 11th volumes of the manga.
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