If you're watching for the , most of the short specials can be skipped as they are purely for comedy. However, the full-length OVAs and the Hero: Taiikukan-ura no Holy special are highly recommended. The latter is especially critical for those confused by the changes made during Season 3, as it helps bridge the gap to the more faithful Season 4 adaptation. How To Watch High School DxD in The Right Order!
A special story distributed with the BorN Blu-rays that offers a glimpse into a potential future involving Issei and the heroines' children. Season 4 ( Hero ) Extras High School DxD Specials
The extra content is generally divided into three categories: standard OVAs (which usually act as "episode 13"), short DVD/BD specials, and specific alternate-story episodes. If you're watching for the , most of