Seen at Subculture:

This OVA takes place some time before the major school festival in the anime series. Oh, how I missed all of the Manabi girls. Being the slice of life fan I am, Manabi Straight has been one of my favorite anime since its airing earlier this year.

It probably could stand reminding that Manabi Straight is not a "slice of life": it is driven by a strong plot. However, the OVA is. It's the only filler episode in the series, basically.

The border is somewhat fuzzy. For example, Kamichu has a story too, only it hides most of the time and lets slice-of-life to come to the forefront (e.g. ep.12 in DVD count). Therefore, Kamichu is slice-of-life or primarily slice-of-life. But Manabi isn't.