In my opinion, the OVAs sure are well produced. The other characters are given screen time so viewers would know them better. In addition, the ending of the OVA is superior to the one in episode 12. Although Harekaze sunk, it will continue the journey with the students in another form.

I said the ending is good enough to conclude the story because I believed that the story did end "properly". Many other animations which are based on light novels or mangas may be unable to actually "end" the story. If the plot is long, the animation can only tell part of the story (e.g. Accel World, KonoSuba, and possibly Tsugumomo). In a few cases, the scriptwriters may write an ending to the animation, while the story in original work goes on. This animation is a TV original one, which means the scriptwriters write the story so that it fits into 12 episodes (and the OVAs). However that also means for a 2nd season, instead of having something (light novel, manga) for reference and making modifications based on it, the scriptwriters have to write the story from the very beginning (i.e. after the students are given Y-469).

Therefore I do not want to be too optimistic. That is, the higher your expectations, the greater your disappointments. Anyway, you may wish to show your support by purchasing BD/DVD. After watching OVA 01 I find the story good and made the decision to purchase a disk, and luckily the ending is up to standard.