Oona Lehmann played her junior career in her native Switzerland. In 2009, she placed third at the Swiss junior women's championship. The following year, she was a member of the national junior championship team. She played third for the Swiss junior team at the 2010 World Junior Curling Championships. The team was skipped by Manuela Siegrist. They placed fourth. Later that year, Oona Lehmann moved to Germany.
Oona Lehmann played lead for the German team (skipped by Rainer Schöpp) that won a bronze medal at the 2010 European Mixed Curling Championship. After that, Oona Lehmann joined Schöpp's sister, Andrea's team at the third position.