The first OAD (original anime DVD) for That Time I Got Reincarnted As A Slime was orignally slated to be released in a bundle with the manga;s 11 volume in May but the OAD was delayed until December 4. As such, the second OAD, released as an extra for the 12th volume manga, (technically) became the first as it was released in July. However, the OAD that was originally slated for release in May will not be released on December 4 and both Crunchyroll and Funimation have confirmed that they will simulcast the new content on the same day.
Crunchyroll's description for the OAD is as follows:
Mikami Satoru, an ordinary businessman, was stabbed on the street and died, but found himself reincarnated in a parallel world... in the body of a slime! Thus he began his new life as a slime named Rimuru, flung into a new world teeming with creatures of many different races, and made the decision to start a new country where all races could enjoy life together.
In the midst of his everyday life, Rimuru suddenly realizes that there's nothing to do for recreation in Tempest. He suggests sumo wrestling as a way for everyone to have fun together, and his friends are immediately on board with the idea. Thus begins Tempest's first sumo tournament, in which everyone competes enthusiastically in hopes of winning the grand prize! Passionate contests break out not only among Benimaru and the men, but also among women like Shuna and Shion, and even Rimuru ends up joining in! This is a special episode in which all the citizens of Tempest come together for some fun.
That Time I Got Reincarnated As A Slime started out as a light novel series written by Fuse, and illustrated by Mitz Vah in 2013. A manga adaptation (which is ongoing and currently at 11 volumes), written by Fuse and illustrated by Shō Okagiri is published in Kodnasha's Monthly Shōnen Sirius manga magazine.
A second season of the anime is slated to arrive in 2020.