Kakugo no Susume
English name: Apocalypse Zero
Genres: Action, Horror, Sci-Fi, Ecchi
Type: OVA, Episodes: 2
Aired: Oct 23, 1996 to Dec 18, 1996
Studio: Ashi Productions
Source: Manga
Rating: R+ - Mild Nudity
Synopsis:
A series of natural disasters has reduced the world to rubble, with the survivors doing whatever they must to survive in a world gone mad. But one young boy, Kakugo, gifted with amazing martial arts and a superpowerful suit of armor by his late father, has been charged with making the world (or at least his school) a safer place. But his sister has a matching set of skills and equipment, and she`s on a mission to bring peace to the world... by wiping out humanity! 

(Source: AnimeNfo)