An OVA in the spirit of season 1 but with more focus on comedy. Really, nothing special to mention about it. There's no story at all and it's a typical OVA in that way. They go on two small adventures that has no purpose and seems to be there just to make the characters fool around for fun. It's just that nothing at all makes sense and it feels half-arsed. It was much better than season 2 though. At least it was slightly humorous.
It was a fairly nice OVA to end this whole story with, managing to rinse out at least a little bit of the bad taste from season 2. You can watch this after season 1 or season 2 as there's nothing going on here and it has no relevance or connection to the main plot whatsoever. I personally assumed it was meant to be watched after season two because it was aired later than season 2, but the truth of the matter is that it's more fit to be watched after season 1 because it's in the same spirit as season 1 and if you watch it after season 2, you go a long while back in time to when they were still running around with the previous teams and it just feels out of sync. If I had to guess when this OVA actually transpires, it'd be somewhere in season 1, somewhere around the middle of that season, at least before the last battle in season 1 because characters are present that wouldn't be there otherwise.