Before we answer the question, it’s important to know what canon actually means. Canon means that it is “officially” part of the main story. For anime, canon episodes and movies should be drawn or illustrated by the current creator of the series.
Filler on the other hand is content not part of the official story. It is generally written or drawn by people other than the original creator and is intended to entertain fans until new official episodes are released.
The Overlord movies are: Overlord: The Undead King (Feb 2017), Overlord: The Dark Warrior (March 2017), and Isekai Quartet The Movie (June 2022).
The first two Overlord movies, The Undead King and The Dark Warrior, are canon to the series. Isekai Quarter the Movie is NOT canon. Isekai Quartet is a crossover movie between Overlord, KonoSuba, ReZero, Youjo Senki, and Shield Hero.
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Are Overlord Movies Just Recaps?

While the Overlord movies are canon, and NOT filler. It might as well be. Overlord: The Undead King and Overlord: The Dark Warrior are both recap movies. It is not necessary to watch these movies and you won’t be missing anything if you skip them entirely.
The Overlord movies are only worth watching if you plan to skip the first season.
The first two movies recap season 1 of Overlord. So if you’re looking to skip season 1 and go straight to season 2, you can save some time by watching the movies instead. Personally, I recommend watching the first season. It is well made and provides the best viewing experience.
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Overlord Movie Watch Order

The watch order for Overlord movies is as follows:
1 – Overlord: The Undead King – (Feb 2017)
2 – Overlord: The Dark Warrior – (March 2017)
3 – Overlord Season 2-3
4 – Isekai Quartet The Movie (not canon)
You can easily substitute the first two movies with season 1 and vice-versa, depending on whether you want the full viewing experience (series) or a quicker viewing experience (movies).
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When Does the Next Overlord Movie Come Out?
Overlord: The Holy Kingdom was announced on May 8, 2021. It is the next Overlord movie to be released. It will cover the Holy Kingdom arc from the light novel and follows the events of season 4.
We expect a release date to be announced shortly after Season 4 is completed.
So far, Overlord season 5 isn’t confirmed. It’s likely that season 5 will take place after the events of the Holy Kingdom movie.
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