You can find the death certificate at "Ancestry". He was born in Köslin and died there on May 12, 1909, at the age of 57.
His parents last resided in Köslin: the master shoemaker Carl Müller and his wife Friederike, née Herrmann.
Otto Müller was survived by his wife Agnes née Mews.
This could be their marriage (unfortunately the link to the Polish page in the database does not work for me):
www.pommerscher-greif.de
If so, he was born on January 8, 1852 in Cöslin (=Köslin) and his wife on December 21, 1859, also in Köslin.
The Greif database also contains information on children of this couple.
Here you will find the death of his father:
https://www.pommerscher-greif.de/datenbanken/greif-index/detaildatensatz-greifx/?id=CD-00064268
Otto Müller, as Vera has already written, is such a common name that you should be absolutely sure you have the right Otto in your family tree with this "office assistant". Good luck with your further research!
Jan