Hi Peri,
1. First you need to extract/replicate OLTP tables from your source system to SAP BW system.
2. Second you need to create infoproviders on top of your extracted data.
3. Now create query by using BEx query designer on top of your infoproviders(InfoCube, DSO, multidata provider).
Hope this might help you. 
Best Regards,
Ashish