Hi Christian.
In the overview
page for the Microsoft Office Specialist (MOS) Certification you can read that you have to "Pass any of 5 exams to become certified in that program." (quoted).
If you want to get certified either as Microsoft Office Specialist Expert or Microsoft Office Specialist Master, you have to pass more than one certification exam, as you can read
here and
here.
Bye.
Luigi Bruno
MCP, MCTS, MOS, MTA