Otso Rantakari arrived in Davos at the beginning of the 2019-20 season as an injury replacement for Magnus Nygren, who had injured his knee. Moreover, he spent part of this past season on loan with EHC Biel, who needed a temporary replacement for Anssi Salmela.
In 30 games with HC Davos, Rantakari scored nine goals and put up 13 assists, while he collected four points (two goals) in seven games with EHC Biel. His eleven goals were the most by any defenseman in the National League this season.