So... is Ojisan to Marshmallow finished or not?
NO, it is not finished.
Why?
The anime finished before the manga:
Anime finished: March 25, 2016
Manga finished: February 15, 2019
MangaUpdates contains information about the anime adaptation and indicates that it is incomplete.
Watching Order (by release date)
Here you have all the anime relations we detected ordered by release date, so you can see the reason for this status in context and also use it as watching order.
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Synopsis
Ojisan to Marshmallow is a short-form comedy anime about Habahiro, an older office worker with a deep love for marshmallows, particularly the Tabekko brand.
His younger colleague, Iori, finds amusement in teasing him with his marshmallow obsession, often playfully depriving him or using them to get his attention.
Unbeknownst to Habahiro, Iori's actions are part of a larger, subtly romantic pursuit. She seems to be drawn to his gentle and marshmallow-like qualities.
The anime follows Iori's increasingly blatant attempts to win Habahiro's affection, all while Habahiro remains largely oblivious to her true intentions. It explores their quirky workplace dynamic and the humorous situations that arise from their interactions around marshmallows.